I use music from my personal library, all of it's copyrighted. I monetized my channel years ago but I don't make anything off it, wouldn't be much $$ anyway. All the proceeds from the ads go to the artists. That's fair to me, only a a occasional CR constraint and I get to use most any music I want. There's a few artists that aren't in that program but it's rare.
There's paid option which have great music with no CR but you'll pay: Epidemic Sound – Royalty free music and sound effects
YT Offers royalty free music also and there some other free options.