which is best free music download app for mac

By free music, you mean legal music? Sorry I had to ask, but music is usually paid for and not free. Most download sites give free tracks, including iTunes - Saint Cecilia - EP by the Foo Fighters is Free on iTunes at the moment all over the world 😁

Google play also offer free tracks... so do what the rest of us do, pay for it. 😉

In general, free music is not a thing.

The people who make the music deserve to be compensated for their work, just like everyone else gets paid for their work.

Using a converter like you mention to convert copyright music is ILLEGAL.

You must pay the artists for their work.

The converter application itself may be free but doing what it does is for most purposes illegal. Given that many of those YouTube music videos are probably pirated music to start with I don't know how they manage to stay up there unless it is simply a matter of them slipping through the cracks.

Investigate other sources. My local library has a Freegal subscription where I can legally download 5 mp3s per week. No, you aren't always going to find the latest hits there but I usually find some I want.