Music comes from a licensed music library. You can pick the track, or I'll suggest a few that fit the cut and you choose.
Popular songs can't go in. A track from the radio or Spotify needs a license from the label and the publisher, and those are rarely available to small productions. If you put one on YouTube or Instagram anyway, the video can get muted, blocked or claimed. Library music avoids all of that.
Bringing your own music? Send the file and tell me where it came from.
Not sure what you want? Send a video whose music you like. I'll find something with the same feel that you're allowed to use.