@KontraVersus You can always try to look at the licensing of the song if they present such information.
https://creativecommons.org/share-your-work/cclicenses/ Has some knowledge it seems, but I have no idea how it works exactly so I can't really help in understanding any of the specific licenses.
https://freesound.org/ This website is cool as it has songs you can use, although some have instructions on how to use them, so you have to look out for that.
Cushee-Foofee
Just because you get away with doing something on Youtube doesn't mean it's legal, just that no one wants to bother going through the legal process if it's not something too big or serious.
KontraVersus
I dunno how that works, hence the question. Thought that it might come under fair use if it's just an artistic non commercial project