I've settled over the years on using syncthing to keep copies on my machine and phone. After testing quite a few android players I settled on Oto Music. I'm currently back in the Mac world and use Cog. I still like to use cmus in the terminal.
For acquiring music, the bulk I get from bandcamp. I usually try hard to get my music legally, but sometimes it isn't possible, ripping from YouTube, searching torrent sites, using russian search sites which still seem to index the blogs that stored downloadable links to music like we had about 10 years ago...
Yes, synchthing is godsend + also there are several music streaming docker based servers you can pair with an app on your phone and stream only the things you like (or download them) without moving the whole collection on the device.
For acquiring music, the bulk I get from bandcamp. I usually try hard to get my music legally, but sometimes it isn't possible, ripping from YouTube, searching torrent sites, using russian search sites which still seem to index the blogs that stored downloadable links to music like we had about 10 years ago...