Open the Photos app
Select the video
Tap Edit if available
Use the Live Photo or Loop feature if the video was created as a Live Photo
For regular videos, open the video in the Files app, iMovie, or a third-party looping app
In iMovie, import the video into a project
Duplicate the clip on the timeline as many times as needed
Export the repeated video
Use the repeat or loop option in a video player app if available
Use Shortcuts to automate repeated playback if needed