The other responses have some great answers, but I learn best by breaking something that is already working, and so this list may have useful sources of code to poke around in: https://github.com/serhii-londar/open-source-mac-os-apps
That list is an invaluable resource, but it's huge and a bit daunting.
If you're interested but not sure which project to start with, I recommend looking at NetNewsWire[1]. It's a very nice codebase with experienced Mac and iOS developers on the team. It's cross platform (iOS, macOS, ipadOS), uses the latest technologies, and has speed and stability as design priorities.
It's the opposite of an Electron wrapper around a web app.
If you're interested but not sure which project to start with, I recommend looking at NetNewsWire[1]. It's a very nice codebase with experienced Mac and iOS developers on the team. It's cross platform (iOS, macOS, ipadOS), uses the latest technologies, and has speed and stability as design priorities.
It's the opposite of an Electron wrapper around a web app.
[1] https://github.com/Ranchero-Software/NetNewsWire