I learned tons of new stuff... mostly that putting an App in the Apple Store is a lot of screen shots and image creations to make a good first impression - it's Marketing / NOT coding. So you have to put on a very different hat and get creative. I enjoyed it - but it's not my every day cup of tea.
I learned that the SwiftUI framework is ready for prime time and makes App Dev real easy (ok, much easier than UIKit). I'm now learning to Test Drive my App Designs (TDD) and can teach you how if you want to follow along https://www.theswiftdojo.com/videos-to-watch