You are only “allowed” to use what is in the YouTube library that another mentioned as those audio are “free” to use. If you play background music it will get flagged for copyright as you dont have permission and it will make the video ineligible towards moneitization. That goes for shorts as well. Even adding a sound on it for your short can cause the same problem. As it’s for attention in a sense. At that point it’s more than just about engagement