Spotify seems like it has a lot of music on it, until you close Spotify and look literally anywhere else there's music.

Netlabels are especially fun to explore, and you're going to find extremely incomplete collections, if any at all, on most major streaming platforms.

Archive.org is a great music source for netlabels.

SonicSquirrel is an excellent collection of various copyleft music.

SceneSat has amazing chiptunes, and that's actually something you can (mostly) listen to on Spotify now.