It’s extra sad, because you used to be able to put your own MP3 files in Spotify. If I recall correctly, it would even sync those files to other devices!

But this is long ago now, I might be mistaken

You aren't mistaken. I used to use Spotify for that purpose as well.

I had an account with Spotify premium, which allowed you to download playlists to listen to offline. On my computer, I would select a local song to add to a playlist, and on my phone I downloaded it to listen to.

I believe it still does that, but cannot confirm, because the way it syncs the stuff is kind of aligning the stars in a specific way. I've followed instructions and it never seemed to work quite right. It skips some, it dupes some, some might only work on the same network (even when everything is marked for offline)

I do have local stuff (mp3 based) on my Spotify still on my iPhone. Not sure if it is a relic of an old feature. But it's not the entire directory and it's not easy to manage.

Yt has year in review as well now I think. As far as the interface yt seems like a complete clone of spotify