If I have the premium, how to download videos of that quality?
I’d guess yt-dlp (1) + cookies from your browser or Google username and password. You can run yt-dlp on iPhone in a-Shell (2).
a-Shell supports Apple Shortcuts, so you could, for example, make a shortcut that runs ‘ yt-dlp -x -f bestaudio -o "% (playlist_index)s - %(title)s.%(ext)s" ’, which downloads a playlist and maintains the order (e.g 1, 2, 3 and so on).
On a side note, a-Shell can help you get around tethering limits using iOS-SOCKS-Server (3) or an equivalent. On Android, PDANet is great for this, but on iOS it takes a little more work.
1: https://github.com/yt-dlp/yt-dlp