best video, best audio, MKV remux:

yt-dlp -f ba+bv --merge-output-format mkv $URL

best video UNDER 1080p, best audio, MKV remux:

yt-dlp -f "ba+bv*[height<=1080p]" --merge-output-format mkv $URL

change output name from default:

yt-dlp -o custom-name.mp4 $URL

video to mp3:

yt-dlp -x --audio-format mp3 -f bestaudio $URL