There was a lot of effort involved in recording app preview videos - they have to be recorded separately on two different iPhone sizes and I don't have the more modern one.
The workaround was supposedly to record via the Simulator but that turned out to have its own woes - some interactions were not reflected in the video (eg: an image being updated). I spent over an hour recording a 20second clip in multiple takes, working out what else I could do to trigger getting the final refresh.
After that, the recorded videos are unusable directly - they are 60fps twice the speed of the requirement and are a different resolution. Apple include a special mode in iMovie - create an App Preview project, import your movie and then export as App Preview. That magically fixes speed and size.
Apart from all that, make sure your video content shows the app in normal use and doesn't show others - I had two "Metadata Rejected" issues and a week-long review.