Foobar2000 has an andoid app that works a treat. You'd be hard pressed to find a mobile phone that doesnt have enough space to have a folder full of enough music to ocupy yourself.

foobar2k on wine running on an old-ass lenovo

in seriousness, I don't know what to recommend on mobile. I'm using gonemad media player classic from 10 years ago because my phone is old and I don't like exploring new apps

Poweramp is pretty good, although it's not free (maybe there's limited functionality for free, can't remember).