It's up to you but exactly as you say, talk about the actual technolgies you used i.e. this list is a good starting point and how that relates to your app. E.g. I used CoreML in order to work out what type of recycling bin an item should go into using a dataset I made with objects around my home. (try and extend this out a bit)
Also put a bit about your project and what it actually is but the main focus should be on the Apple technologies that you used. I recommend writing it in a separate document and then pasting it in in case you want to resubmit and to not need to do it all in one go.
Hope that is helpful,
Freddie