Hi, what's your setup?
I often listen to music through youtube on my phone connected to a bluetooth speaker. I use Newpipe, works very well. Then when I want to save a song or an album, there's the option for downloading (in newpipe itself) or on android for example Seal (works really well for downloading entire playlists and unselecting some sponsored video's from the playlist).
The hassle is uploading from the phone to my jellyfin. I've used File Browser, bit limited in options.
Then I thought I could use Syncthing to have some folder from my Android phone upload it automatically to my Jellyfin server (pc running dietpi), but it seems Syncthing is now discontinued on Android?
What I was first looking for was my own hosted yt-dlp with a mobile friendly UI, but that seemed quite difficult to get running.
for uploading, absolutely use syncthing. you can set it up so that it works in only one direction, i.e. phone to server, so any file that appears on the phone's download folder gets sent to the server. the one you want is syncthing-fork on fdroid.
as to listening to the music via youtube check out innertune, also on fdroid.