Taken from here

yay -S atomicparsley

youtube-dl --extract-audio -f bestaudio[ext=m4a] --add-metadata --embed-thumbnail YOUTUBE_URL

Update 2025-12-11

yt-dlp is the successor to youtube-dl. Here is the latest (with an appropriate cookies file)

yt-dlp --audio-format m4a -x --embed-metadata --embed-thumbnail --cookies cookies.txt YOUTUBE_URL

You can also do a list of URLs by passing the -a argument:

yt-dlp --audio-format m4a -x --embed-metadata --embed-thumbnail --cookies cookies.txt -a playlists.txt

I used this cheat sheet to get the parameters I want.