I just released version 2 of Zavala. Zavala is an open source project. It is written in UIKit. The Mac version is a Catalyst app and I think a good example of how you can make a good Mac app using Catalyst.

The project uses a lot of modern Apple API's. It syncs using Cloud Kit. The outline editor is implemented using UICollectionView. It uses the new 3 panel UISplitViewController. The code isn't perfect and there are hacks in there to get around UIKit weirdness and bugs. You can use it either as a resource to learn what to do or what not to do. :-)

If you just want to try it out, you can download it from the App Store.