Open the iMovie app , tap Start New Project , then select Movie . Choose your clips (like "IMG_0124.MP4") from your library and tap Create Movie .
Most smartphones have native "merge" or "movie" features that don't require downloading new software: IMG_0124MP4
In your Gallery app, long-press to select multiple videos, tap Create , and select Movie . This will automatically arrange them on a timeline for editing. Open the iMovie app , tap Start New