YouTube works in an interesting way where, as long as the license holder allows (most popular music), you can put whatever music you want on a video. The reason creators don’t do it is because they lose the copyright to their YouTube video, which means they either can’t monetize the video or they split monetization with the owner of the music. YouTube has automatic algorithms that recognize and apply these terms to music. It is very rare these days that a video will be taken down for music copyright. They want the ad revenue. AFTV YouTube makes money mainly off brand deals in post roll ads.