Here’s an odd one… there are at least two ways to get an MP4 clip into iMovie on your iPhone (and presumably iPad). I’m not talking about moving a video so you can watch it with the videos app, but into an iMovie project.
The first way is to use the app-specific storage in iTunes, but I needed something faster and more generic. So the second way is to put the clip into your iCloud Drive storage, then iMovie is supposed to be able to add it to a project.
The first time I tried this, it just didn’t work. The folder list in Drive didn’t even match the list in iMovie! Adding a folder to try to force a refresh made no difference.
So here’s the fix: Take a quick movie clip on the iPhone, bring it into iMovie and save the movie (or project) in iCloud Drive. Magically, an iMovie folder gets created in Drive and it all starts to work!
Hope this helps you.