How to extract a video from Youtube and optionally to extract mp3 from the video. On Ubuntu 20.10:

Install youtube-dl (for downloading from youtube):

sudo curl -L https://yt-dl.org/downloads/latest/youtube-dl -o /usr/local/bin/youtube-dl

sudo chmod a+rx /usr/local/bin/youtube-dl

Install ffmpeg for sound extraction:

sudo apt install ffmpeg

How to extract video (note – has to be your paths to python3 and youtube-dl):

/usr/bin/python3 /usr/local/bin/youtube-dl https://www.youtube.com/watch?<youtubeid>

How to extract sound:

ffmpeg -i '<title>-<youtubeid>.mkv' '<title>.mp3'

With thanks to 3 Easy Ways to Download YouTube Videos in Ubuntu and Other Linux Distributions; How to Extract Audio From Video in Ubuntu and Other Linux Distributions; and Youtube-dl: Python not found (18.04)