Using youtube-dl -f 251 $url Will download the ~160kbps opus audio if available, but it comes in a webm container, so I'll use something like ffm

Using youtube-dl -f 251 $url Will download the ~160kbps opus audio if available, but it comes in a webm container, so I'll use something like ffmpeg -i "$file" -vn -acodec copy basename -s .webm "$file".ogg to repack it (without re-encoding) in to an ogg container for playback on Android. Many talks and lectures etc. could withstand the lossy transcoding to an even lower bitrate than Youtube offers (-f 249 = ~50kbps) but space isn't that much of a concern.