Huh. I had completely written off that use-case because literally any time I've wanted to do anything at all, I've found that whatever movie editing software I'm trying at the time can't do it. So instead of wasting time by trying to edit on iOS, regardless of how simple what I want to do is, I always just copy it to my laptop or desktop and do the editing with real software on a real computer.

But maybe there exists better options. I can't say I have invested in expensive video editing software on iOS. (Not that I've ever had to on macOS or Linux though.)