Can't Apple just do this?
How do I add music to a video I took on my iPhone ? And then post on IG
iPhone 11 Pro, iOS 13
You use the iMovie app on your iPhone to edit the video and add music to it. I don’t believe InstaGram will let you post any videos with copyrighted material (music). I know that YouTube does not allow that either. If you have music that is not copyrighted, then you can add it to your video in the iMovie app and post it to social media.
Axel F.