How to get a Youtube song into MP3.

"4K Video Downloader" will let u do what u want

http://mp3fiber.com/

Online converting

Google "youtube to mp3 converter" and you should get a bunch of websites that will let you paste the youtube link and then download the mp3 file. Sometimes the sites stop working/can't be used in your country, so try another one. I've been using https://www.grabfrom.com/ most recently because there are options for trimming off silence at the beginning/end, increasing volume, etc.

add "ss" before youtube link (example www.ssyoutube.com/randomvideolinkcodethingy) it'll bring you to a savefrom site I think it's called, and I believe there's an option to download the mp3 version of the video, in addition to downloading the whole video in mp4 format.
I sometimes download videos for offline viewing and I believe I saw an mp3 option somewhere