okay, since I really don't understand this stuff: My most popular YT video is my performance of a famous marimba solo. Can I monetize that?

(I did everything EXCEPT compose the piece)

The answer is technically that you should receive the performance share of money and the composer should receive their share.

Yeah, I should get a Copyright notice, right? I have a few videos "sharing revenue" in a similar manner. I'm just afraid of losing the vid

Use a service like adrev in order to collect on the track, and many composers don’t.

One thing you can do is contact the composer or publisher and ask. Like I’m pretty sure materia collective doesn’t use a service like that