The source code for NetNewsWire might be a useful starting point: <a href="https://github.com/Ranchero-Software/NetNewsWire" rel="nofollow">https://github.com/Ranchero-Software/NetNewsWire</a>
I can highly recommend Paul Hudson's courses, eg, Hacking with macOS -<a href="https://www.hackingwithswift.com/store/hacking-with-macos" rel="nofollow">https://www.hackingwithswift.com/store/hacking-with-macos</a>. They are heavily Swift-focused by very good.
I’m kinda bummed that you only got two replies in total... Mac development is a little bit of a lost art these days. There are some very skilled people out there who knows it. But not so many who choose to learn it though these days. All focus is on mobile...
Take a look at Jordan Singer's SwiftUI apps. All are open source, most are macOS apps.<p><a href="https://ibuildmyideas.com" rel="nofollow">https://ibuildmyideas.com</a>