To download an mp3 from youtube all you have to do is change the link in the address bar slightly.
When watching a youtube video on a computer change the URL in the address bar
If instead you have a link that looks like this:
Change it to:
For mobile URLs the change:
Alternatively you can bookmark the following bookmarklet:
💾 SAVE SONG️
To do so drag and drop it onto your bookmarks bar
Click that bookmark whenever you are watching a video (song) you want to download and it will download the current video (song); consider it a download button.