Do I use a website to download songs off of YouTube or Spotify?

Where should I store the music? I haven’t any clue about self-hosting. I’m running GrapheneOS, is it enough to save the songs in Files and play in an app like Auxio? Maybe sync with SyncThing?

What’s the best way to compress mp3 files but still retain the quality (even possible)?

Could really use some help as I’m very inexperienced. :)