Open the Photos app
Select the video you want to edit
Tap Edit
Tap the Music or Audio option if available in your app
Choose a song from your music library or available audio
Adjust the audio volume if needed
Save the edited video
Use iMovie if the Photos app does not support music adding
Install and open iMovie
Tap Create Project
Select Movie
Choose the video
Tap Create Movie
Tap the plus button
Select Audio
Choose Soundtracks, My Music, or Files
Tap the song you want to add
Adjust the audio length and volume
Tap Done
Tap Share to save the video
Use CapCut, InShot, or similar video editing apps
Import your video into the app
Add music from the app library or your files
Trim the music to fit the video
Export and save the final video