I think it might be called a "Manheim Crescendo" or "Manheim Rocket", but I'm not exactly sure...

That's a gradual increase from very soft to very loud with a repeating figure over a pedal point, so it kinda sounds like what you're looking for? :)

The only one I can currently think of is the sonic 2 boss music. It's very simple and loops ,a few times, but every time, More instruments come in.

I really wish I could think of the other one, because it was such a good example of what I mean. Aaaah.

Here's a other example https://youtu.be/Mbby7d4VGkM

OKay, that one sounds more like Terraced Orchestration, where they gradually add in more instruments as it repeats. :)

Google isn't really showing me much for that term

That's because I was overthinking your question... XD

It's a Round...

That's where a particular piece of music repeats and more and more instruments/vocals are added with each pass.

My brain hurts... XD

Its just called "Progression", people will say "the song has excellent progression as it builds" for example. The looping bit is irrelevant if its the same song.