I just use what ever music I think goes with the video. YouTube normally allows it but puts ads on the video for the song owner.
Alternatively you can use the songs that YouTube now provide or from one of the various copyright free music channls such as NCS (NoCopyrightSounds) etc
Alternatively you can use the songs that YouTube now provide or from one of the various copyright free music channls such as NCS (NoCopyrightSounds) etc