If you want more creative control (transitions, effects, or specific aspect ratios), these apps are highly rated: Merge Videos: Combine Your Videos For Free | Adobe Express
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 . IMG_0124MP4
Most smartphones have native "merge" or "movie" features that don't require downloading new software: If you want more creative control (transitions, effects,