There are several Q&A threads that explain how to download youtube videos using the terminal.
However, I would also like to learn how to extract the video's soundtracks as MP3 files--also using only the terminal.
Answers briefly explaining how to use youtube-dl
or other similar utilities before explaining how to extract the MP3 would be ideal for the sake of having all the information in one place--even though this aspect has been covered in other posts.