It’s the next note you play that decides if a note works 
Do something wrong once, it’s a mistake. Do something wrong three times it’s art!
Pentatonic box scales are useful. Just do NOT neglect the major seventh when you’re playing in a major key. Using a well-placed major seventh is key to playing great pop music. A lot of the classic pop acts like the Beach Boys often used the major seventh in the background vocal harmonies. Carole King used them on the piano. John Mayer used the major seventh interval effectively in his opening intro to “No Such Thing”, as did the Smashing Pumpkins in “1979.”
I cannot begin to count the number of people, including some very good self-taught musicians, who don’t know what a major seventh is. Once I’ve sat them down with a piano or guitar and demonstrated what it is and how to use it, it blows their minds. “So THAT is how to get that ‘dreamy’ or ‘wistful’ sound in a song!! I’ve always recognized the sound, but never knew what it WAS exactly.”
It’s really a matter of taste and context. In many rock contexts, I think major sevenths sound cheesy or inappropriate. I find the minor seventh much more userful. But, like I said, context. It does have a “wistful” and even jazzy sound to it. It works great on “1979,” for instance without making the tune sound too “easy listening.”
I am surprised, though, at how many musicians you’ve met that don’t know what a major seventh or major seventh chord is. It is a pretty basic concept, so every musician should know it but, like I said, it is not as readily encountered in the rock context, so I guess I could see someone being unfamiliar with it. But that hasn’t been my experience.
If you are playing a kind of music that has jamming, then at least 99.875% of the time you are doing functional harmony. Some sub-percentage of that, you may be doing functional harmony badly because you think you’re doing something else.
Most people that I colabborate musically with, come from a rock background, stuff like Weezer, the Smashing Pumpkins, the Pixies, the Flaming Lips, etc. Many of them are self-taught or if they took guitar lessons, only learned chords or rote scales without really learning the theory behind it. Lots of these kind of people are really, really good at playing music, but have NO vocabulary to talk about theory. I don’t really play jazz, just rock/pop, though I DO come from a jazz background as much as a rock one.
All the stuff I mentioned above is saturated with major sevenths, major sixths, in some cases ninths and tenths and thirteenths. (Remember that iconic riff at the beginning of Scar Tissue? It’s Mr. Flea playing an interval of a major tenth on the bass.)
But lots of people don’t have the vocabulary to talk about this stuff. They know it intuitively though, which is why it’s easy for me to TEACH them about it.
OK, that I can see. I’ve been lucky to play with musicians who know how to name basic chords, as a lot of them do play from chord sheets/tabs where the chords are named and their fingerings are given. And of course, I am also familiar with just playing “shapes” on the guitar and not knowing exactly what the chord is (hell, when I play guitar, I often dick around with alternate tunings and drones and have no idea what chords I’m playing unless I sit down and actually think about it.) I just would not have thought a major 7 chord is that far outside the vocabulary of most guitar players.
It’s weird, but it’s not taught often in rock guitar until you are past beginner stages, which means self taught people almost never learn them. I do not know why, since in many cases in the open form they are easier to play. Makes no sense to me, but that’s how the curriculum was when I was learning way back when and my daughter is going through the same situation now. It is a different chord shape for guitar so it’s, I suppose, another thing to memorize as a beginner (unlike on piano where it’s just add an extra key.)
It’s a weird guitar centric quirk, but it’s common.
I’m guessing it’s because it’s just not that commonly used in rock (unless you’re playing softer or more mellow styles like “yacht rock” or jazz rock.) That said, you’ll find it in occasionally in harder rock, too (“Plush” by Stone Temple Pilots comes to mind, and they use a couple other funky chords in there, too, by rock standards, like a diminished 7th.) But typical rock is heavy on the blue influence/blues scale where the major seventh clashes.
At any rate, OP, work your way up to understanding that kind of stuff. You’ll be fine for a long time just learning your “boxes” (minor/major pentatonic and blues), and jamming over simple chords. When I improvise in a rock/country/blues context, I almost never end up using the major seventh–I don’t think it’s that important for this style of music, but in other contexts it is useful/essential. Just start with the five or six notes and get comfortable with making up melodies on the fly.
I’ve jammed with guys who played their asses off and played every chord that they were supposed to play in any given song - even relatively complex ones like Radiohead songs. But they had no idea what the word interval meant. They had no idea what a mode is. No idea what Lydian and Mixolydian is.
Guys who have studied the Dead a lot tend to at least understand basic modes because the Dead frequently used the Mixolydian and Lydian. A lot of jam band musicians that I’ve encountered seem to have a background in jazz, so they usually know what’s up. Anyone from a funk or blues background likely also understands some of this stuff. Power-pop or alternative rock influenced players can kill it in their respective idioms, but have no idea what a major seventh is or even the concept that minor chords can be in a major key because the notes comprising the chord are notes of the major scale of the key. But since they often have been listening intently to music their entire lives, if I sit down with a piano for 10 minutes with them and explain some of this stuff to them, they grasp it instantly. “OOOHHHHHHH!!! I see!!! I see!!” I always use “1979” and “No Such Thing” by the Smashing Pumpkins and John Mayer respectively to illustrate the major seventh. Always use “Lime in the Coconut” to illustrate the dominant seventh.
That all perfectly normal. Many of the people I’ve played with didn’t really know what a mode was, either. I get what you’re saying. I was just a little surprised about something that you’d encounter in a chord chart not being well known but, as I’ve said a couple of times, as that particular chord is not super popular in rock music, I can see not knowing it.
That chord comes up all the time in rock music though, but rarely in “hard rock”, certainly not in metal, probably not much in punk. But it will be used liberally in anything bearing the influence of the Beatles, the Beach Boys, the Grateful Dead, the Allman Bros. (Melissa), tons of British pop rock music from the 60s and 70s (Al Stewart’s Year of the Cat opens with a splashing dive into a lush Maj7 chord after the piano intro), James Taylor, Jackson Browne, through to more current acts like John Mayer or Dave Matthews, those major seventh chords are all over the place in the general repertoire of pop music from the past half century.
Knowing how chords work (either getting to know through experience, or formally learning from a teacher who you can understand) is far far more helpful than learning a certain chord. Though learning a certain chord can make a big difference too.
By “how chords work” I really mean just the basic harmony and musical structure stuff that most songs have in common, so you can start to predict what chord might come next and why, and so that you’re appropriately surprised when someone messes with harmony or song structure - instead of being surprised all the damn time. 
That sounds exactly right to me. Which is partly why I’m surprised guitarists apparently don’t know it, although I’m trying to come up with a reason, which is that they are playing more blues-based rock kind of stuff. I stand by my assertion that it’s not “super popular” in rock (which I do mean the more blues based styles, as I say before “unless you play softer or more mellow styles.”) It’s a very “jazzy” sounding chord. I know in the last band I played and recorded with, we had a song that I had to play piano on, and in the chorus, I threw in a major seventh, and I thought the singer/songwriter was going to punch me because it all of a sudden gave the song a bit of a jazzy feeling. I thought it sounded great in context. In the end, he left it in, but I’ve always felt like he thought the major seventh turned it into some adult contemporary number. I thought it added interest a la a Badly Drawn Boy song.
And I thought I had overthought stuff. My post is just practical jam tips. It’s long but it sets up someone with no experience in an easy way.
Dissecting how a maj7 fits in?! Seriously, who the fuck cares.
Piano players.
Edit: also bass players. And jazz. They all make fun of us.
I know I should keep my mouth shut, if I simply listen I might learn something and don’t get as easily confused (just realize how very little I know). But as soon as I speak up I tend end up with twelve times more questions than answers. That being said, understanding how to use a major seventh is necessary for me to understand how to play music (which is not scored) which is why I have no understanding of “jamming” and have been trying to ask this question or a remarkably similar question for months.
First, I am not even sure what is being referred to. A Dominant seventh is a chord played on the fifth step up in a major scale that includes the leading tone from the octave below (and may or may not include the fifth of the chord)—correct? A Major seventh would be a diminished chord (three half steps plus three half steps) that is the leading tone but with a fourth note that is either a half or full step below (???) an octave of the note that is the lowest note in the chord?
What these chords are, and how they are used is the entirety of my question “How does one become a musician?” Saying anything like: “Play some combination of these scales over this background and eventually you will get a feeling for it” does me absolutely no good whatsoever. That bottoms-up approach does not work for me (or my kid—who can pick out tunes and recreate sounds on his keyboard, and has ‘learned’ dozens of classical and jazz pieces he has the music for [by learned I mean memorized how to play the right notes in the right order at the right tempo]). I am sorry, but I require more structure and better understanding to do this. I need a top-down approach that explains some relationship between elements. (Something like this: play this chord when you want your audience to be sad… or, end the piece by playing- this, this, this, then a flourish…. Or to make a bigger finish ……. Something more than play notes and listen to hear if they sound good together.
If I may, my brother is a chef and makes a few incredible dishes. He prides himself on the simplicity of his recipes’ and will walk me through them when he is in town. Often you must prepare different elements in different pans, then simply combine them in a certain way to make something much better than I could ever create. But two-thirds of the way through, he will inevitably give me options which fuck the whole thing up. He will say something like: you may add blah, or blah to the chicken if you don’t have duh. Never add it to the sauce though, and you may use any cheese from this region. You have to do this while it is hot---- what are you doing, no that needs to cool before….
Instead of a simple set of guidelines that will allow me to more or less replicate the process, I end up with a lesson on the history of cooking dating from the middle ages that has footnotes explaining how Asian influences changed this or that and what spices come from what part of the world and who first used them. He will sometimes (moments away from putting the food on dishes and actually eating it— easily my favorite part) say: “now you can grab a sauté pan and mix [exotic ingredients never found in my kitchen] in a light oil, it must be extra light olive oil from Sienna…….” And I think, this is what is what you are going to be specific about?? He starts a whole separate process more complicated than the first one, and it makes me never want to cook again. My point is all I hear is: “Carefully prepare the first half of the meal - - - then open your spice cabinet and randomly apply different amounts of different spices to the meal, then try to remember if you like it or not.” I realize that is a bit unfair, but it is true. He would then go into extreme detail about what families of spices compliment – but how the true art comes from knowing how to mix the ones that shouldn’t be used together.
That all reminds me of trying to understand music, I really listen and try to understand. Alas, I am not creative enough to make something from nothing—but I am creative enough to understand concepts and use them to build upon each other. (I did play the background blues and the kid who has a much better ear than I do sucked at improvising over it. I however, have a better ‘feel’ for it and I sounded much better than he did. But it was just random plinking – although in a nice rhythm.) Isn’t there a way to learn HOW to noodle around with a groove? Do I really have to make every mistake in the history of music myself to learn how to play music?
So, do I even understand what a Major seventh is?
I’m going to ignore most of what you wrote and focus on what I think your actual question is.
The answer is, yes. For someone like you learning licks and riffs is probably a good idea. Notes are letters, licks are words, riffs are sentences. Learn other people’s by route until you can play them in your sleep. Don’t just learn ones from guitar players either. Find great trumpet and sax players. Harmonica players. Anyone you think sounds great in an improv genre. Get their solos, pick your favorite bits and learn to play them note for note.
Once you know several dozen that you can play anytime start studying deeper. Dig into the theory and figure out why they work then start writing your own. Transcribe them into other keys so you can see the relationship between the intervals and how those stay the same all over the neck. Put different licks together in different orders. Play them with different rhythms. Emphasis on different notes. Add a few notes to a lick and see what it does. Take away a few. Connect some with random notes in the same key.
Eventually you learn enough licks and riffs that you can start to write your own on the fly, and that’s improv jazz or blues.