I've spent the better part of a day creating the first app preview video for Screenshot++, which I hope to launch in the next week or so.
Record your app in action on an iOS device using Quicktime in Yosemite, import the video footage into iMovie, create an App Preview project and start editing. Pretty simple so far.
I created a short soundtrack for the preview in Garageband and added it to the iMovie project. Export and upload to iTunes Connect. Done, right?
I uploaded the video to iTunes Connect under the "4-inch" tab. Then, I went to the "5.5-inch" tab and the preview wasn't there. Why not?
The docs reveal that you're supposed to upload a video for each iPhone resolution (that's 3). But, I don't have an iPhone 6 Plus, so now what?
After some digging, it turns out that Apple prefers videos in each resolution but will provide one version to all devices when viewing in the App Store if you only provide one. This isn't documented anywhere I could find but I'm trusting the good people on stack overflow.
Why not just export from iMovie in each resolution? That was my thought, but alas, iMovie doesn't give you that option. There is also no reasonable way to use Quicktime's screen recording feature with the iOS simulator.
So for Screenshot++ 1.0, I've uploaded a Preview view each for iPhone and iPad. We'll know after the app launches if the video is viewable on all modern iPhones.