This is not a specific variable that makes the barrel explode, he just called "boom" a variable, and used it to trigger a mouvement to the barrel so it crushes on the floor and blows up. No master list of premade variables.
correct.

I could have as easily called it 'efaiuwhefoquv' as I could have called it 'boom'.
even cooler, you can make local variables, meaning that if you want multiple logic gates, or something like that, you don't need tons of variables.
now all we need are logic (i.e, if variable A > B, then C, elseif B > A then D), and more complex operations on variables.
it shouldn't even be that hard to do it, considering unity has a vast array of functions built in (mathf.pingpong, mathf.abs, etc) that do mathematical functions.