let's get to the point.

you need to have yt-dlp and ffmpeg installed, magick and lame for setting the mp3 cover.

yt-dlp has the -x option, which automatically extracts the audio with ffmpeg. to specify the file format, use --audio-format (mp3, aac, etc):

yt-dlp https://youtu.be/oLnq2SdZvQs -x --audio-format mp3 -o flying_north.mp3

as you can see, the thumbnail is not preserved by yt-dlp:

to get the thumbnail, do:

curl https://i.ytimg.com/vi_webp/$VIDEO_ID/maxresdefault.webp -o file.webp

to convert it to JPEG + crop it to 1:1 aspect ratio:

magick convert -crop 1:1 -gravity center file.{webp,jpeg}

and the last thing, to embed it inside the .mp3:

lame --ti file.jpeg flying_north.mp3
mv flying_north.mp3.mp3 flying_north.mp3 # there's a good chance `lame` will export the file to `FILENAME.mp3.mp3`

so, basically it could be collapsed down to a bash script:

#!/usr/bin/env bash

VIDEO_ID=oLnq2SdZvQs
filename=file.mp3
cover=$(mktemp)

yt-dlp https://youtu.be/$VIDEO_ID -x --audio-format mp3 -o $filename
curl https://i.ytimg.com/vi_webp/$VIDEO_ID/maxresdefault.webp -o $cover
lame --ti $cover $filename

and, yay! it works