YouTube audio quality isn’t amazing, but it’s not that bad if you get the right format. Using yt-dlp, you can get opus audio that sounds way better than mp3. Example: yt-dlp -f 251 https://www.youtube.com/watch?v=dQw4w9WgXcQ
Neither. AV1 if available, if not I download a high quality x264 copy and do my own transcode. AV1 is high quality with smaller file sizes, but isn’t very common right now.
I use Tailscale with my Jellyfin server.