MacOS apps are typically written in either Objective-C or Swift as these are the officially supported languages for the MacOS APIs.
The code in this template is interfacing with the Objective-C runtime but with pure C.
The code in this template is interfacing with the Objective-C runtime but with pure C.
In a highly simplified way you can think of Objective-C as preprocessor which replaces the ObjC syntax sugar with C function calls into the ObjC runtime (it's not how it works in reality, but how it could work).
It’s somewhat doable by hand because Objc is a thin lawyer.
Over 15 years ago I did stuff similar to this project to call some Objc code from a C++ app. Most of it was exposed to normal C APIs but one feature only available in AppKit. It was much simpler to do it this way than figure out how to make GCC or Objc like our C++ or any mess with bridging headers.
I think the move to Swift has made that harder in some ways.
But then again I don’t want to write C or C++ these days if I can avoid it.
It was fun building an app in this a few years ago, but was difficult to keep up with updates to MacOS breaking my code.
There's a number of apps that run on modern Macs, that were written in C, but it is unusual to see ones that leverage the GUI.
That said, it's possible to walk from Boston to Portland (OR), but I'd rather take a plane.
When I first started programming Cocoa, I was always kind of miffed that the happy-path was: "Use Objective-C and main() should just hand-off the reigns to NSApplicationMain()". Kind of like the happy-path on Windows is "WinMain plus a bunch of boilerplate crud".
It's always felt somehow vaguely "correct" to have main() be your actual application entry point and have it manage whatever main loop you need yourself. I see that this project doesn't actually quite get there either. It's the same kind of voodoo: boilerplate code, and then cmacs_void_msgSend(NSApp, sel_getUid("run")); hands over the reigns.
I’m not sure you want to spend your time rewriting what NSApp is doing!
How often do you need to get support assistance from Apple? Just do what you want. If it means using Obj-C or Swift for ease, do that. If it means integrating another language, do that.