(which I probably won’t understand the answer to, but I’ll ask anyhow.)

So the human voice can yell at 100 decibles, more or less (This site gives the “loudest human scream” at 128), so 100 is certainly possible.)

If 2 people yell at 100 decibels, they don’t produce a sound that measures 200 decibels, right? It’s not additive. Are they *any *louder than 1 person alone? If so, why? What’s amplifying the sound once it comes out of their mouths?

And why are stadiums so loud? Why are 10,000 people yelling together much, much louder than 1 person yelling alone?

The decibel scale isn’t linear, it’s logarithmic, so they wouldn’t add. Beyond that I’ll let others with more knowledge take it from here.

Unless they are yelling at exactly the same pitch, which is VERY difficult, even for trained singers, there’s going to be some interference between the sound waves, reducing the total possible volume.

Not just the same pitch, but same phase, too.
Sound is pressure changes in the air. If two people are yelling, some of the high pressure regions emanating from each person’s mouth will coincide, resulting in a louder sound.

Note to OP: doubling the perceived volume is a 10db increase.

There are two answers - depending upon whether the sound is correlated or not.
The ordinary case (uncorrelated sound) is that the power level doubles - and thus the SPL goes up 3db. If the sound sources are perfectly correlated - same frequency and maintain phase relationship - they will create stable interference patterns where the level will vary depending upon where you are - with some points actually nulling the sound out and other points seeing 6db (or more if you have many sources) increase in SPL. This simply isn’t possible with real people, but is a significant factor with design of PA systems.

Stadiums are loud because there are a lot of people. But more than this, when you are in the stadium you are in the near field of the sound. Normally - if you move away from a person the power level of their voice drops with an inverse square law. But in the near field of an extended source - ie a stadium full of people - the power does not drop off. It is invariant with distance. So even if you are a significant distance from any individual, you still receive sound pretty similar as if they were yelling in your ear.
The rule about doubling in sound being 10db is only a human perceptual issue - for a change in level that a human being will judge as being “about twice as loud” you need a 10 db SPL increase - which is ten times the power - not twice.