A few years ago I needed to replace the front bumper of my vehicle due to a fender bender (ok, a bumper bender). A corner of it was hanging low and held in place with a bungee.
The local collision shop quoted a price of $3600.
I looked on ebay and found a new replacement for $750. Sold.
With the aid of a step by step YouTube video, I replaced that sucker my own self.
What have you accomplished, repaired, or learned with the Tube of You?
mmm
I learned how to remove old caulk and neatly re-caulk a bathtub enclosure.
I’ve learned the the secrets to authentic Mexican cooking by watching a real abuela prepare food on her rancho in Mexico.
Just a couple of weeks ago, I used a video for guidance in opening my PS5 to install expanded storage. Wasn’t a particularly difficult thing to do (took about five minutes), but I needed to know the right way to orient the machine in front of me and the correct direction to slide off the case, and a video was the fastest and clearest way to see this.
I’ve done lots of household repairs with a YouTube video playing alongside me. Washer, Dryer, Refrigerator, Stove, as well as many car repairs.
Oh, and lawn equipment! I recently did trouble shooting on our Kubota lawn tractor. Found the problem (gas filters, there are two) and replaced the filters. Running nicely now.
Ah! You’re just SO confident most people have one! … Well, okay, you’re right. LOL
I have several, in fact, but one of the most recent was, “swapping out the BIOS battery in a Lenovo Thinkpad Yoga 11e 4th gen laptop”.
I replaced the control panel on my dishwasher.
Oh man! I don’t know what I would do without YouTube videos. Some of the most useful have been:
How to put together an exercise bike.
How to remove and clean a bathroom exhaust fan.
How to put together IKEA furniture.
How to set up a large pet crate and how to fold it down again to go in the storage box.
How to apply rust inhibitor to a vehicle and how to repaint the area.
How to trim a Japanese Maple.
How to dismantle a record turntable and lubricate sticky control buttons.
Et cetera, et cetera, et cetera.
As you can surmise, I’m not very mechanical. Written instructions confuse me. Actually seeing how to do a task step by step is the only way I can get it in my head.
So much is better in my life because of YouTube.
I taught myself German over a period of years. Really helps when I travel to Germany.
Fixed a number of tech/computer issues that flabbergasted me.
I built a bicycle wheel, spoke by spoke. My rear rim started showing some cracks. I could not find a replacement wheel, so decided to do a rim swap, keeping the original hub. The hard part was finding an exact matching rim, given several very specific dimensions. New rim and a few new spokes in hand, I spent an afternoon with a spoke wrench and a truing stand swapping the rim. New rear wheel rolls as smooth as new and stronger with the better quality rim. Cost to have a bike shop do this - around $350 in labor. My cost - around $100 including learning a lot from YouTube U.
Mostly simple repairs on modern cars. These aren’t the cars of my youth. You know how complicated it is to get even a battery out of a modern car? It shouldn’t be this hard!
Not really a fan of YouTube instructional vids. Certainly not for assembling something that comes with written instructions. My general view is that identifying which vids are BS or inapplicable (the vast majority) and which are useful (the needle in the haystack) is a far harder task than figuring out how to do [whatever] without their help.
Having said that there are two YouTube vids I’ve used to fix things in the last ~10 years. How to replace the keyboard on a particular make & model of laptop. And how to disassemble a part of the dashboard on a particular make/model of modern car to get at the back of the entertainment system to attach an aux input to it.
In each case the only hard part was initially getting the device opened up. Once it was open the rest was trivially obvious. Both the laptop and the dash are designed with hidden fasteners. Both for anti-tampering and for aesthetics.
Even so those 10 minute vids could have been replaced by a single pic of the device with the relevant fastener(s) circled and maybe a sentence or two of instruction. 10 seconds to read and understand instead of 10 minutes.
I was making a Key lime pie. I started thinking about juicing all those limes, and how much work it was going to be. And I remembered how a friend had posted a video showing off her apple peeler, and how much time that saved me.
So I went to Youtube and searched for tips. I found someone showing that half a Key lime fits perfectly into a (clean) garlic press. Bingo. Made the whole thing so much easier.
Ha ha, yeah, I was fishing for the biggest or most interesting cases.
I’ve got many more than just my OP, of course:
mmm
Back before YouTube I always had my car’s Hanes Manual. I took a Ford Escort for inspection and failed due to bad ball joints. They quoted a price, but I didn’t want to spend that much.
With my Hanes Manual I was able to replace them! It took several evenings and was not easy. When I took the car back for my sticker the guy asked where I had the work done. I explained that I did it myself. He was angry because one of his employees was unable to do ball joint replacement work and here I was, a DIY guy doing it.
ETA: Haynes.
My husband could give you a long list. He learns all kinds of stuff on YouTube. I think one of the most important ones was how to build a post and wire fence. We fenced off a part of our pasture ourselves, and are able to repair any that get damaged.
Uh-huh, just what I thought! You’re one of those guys who actually knows how to do stuff. 
There’s a good reason why the “For Dummies” books are so popular. 
Generally, I don’t like watching videos - I’d rather read step-by-step illustrated instructions. But just this week, I wanted to learn a certain technique in crocheting, and the written instructions confused me. So I found a video where the woman used really fat yarn and a big hook so it was very clear what she was doing. In 5 minutes, I had it.
My daughter uses the videos a lot for her crocheting - she doesn’t learn from reading instructions as well as I do, but show her and she’s got it. It’s interesting how differently our brains are wired. Well, interesting to me anyway. 
I also learned how to put my long hair up with a chopstick.
Lots of car repair stuff, axil shaft, CV joints, wheel bearings, leaky windshield, tie rod ends, ball joints, string alignments for toe adjustment, beading and breaking tires, parking brake cables, diagnosing check engine stuff, almost replacing a head gasket, though the real problem was relieved along the way and much more. Unless I actually have done it already I usually check there first just to get an overview of the process and any tricks, or in some cases other places to look for a different issue that I may think is something, but really something else.
During COVID I ordered clippers from Amazon and watched a few YouTube videos. I cut my hair (a fade) and beard. It turned out great, but was a pain in the ass, s o I no longer do it myself.