That information is already available, and much of the implementation (if not all) is open source.

You’re free to produce your own toolchain, developer tools, IDE, etc.

This generally isn’t done because it’s a significant amount of work, and you’ll always be playing catch-up to Apple’s comprehensive integrated development stack.

The fact that you need a MacOS machine even to build Xamarin or React native apps always struck me as bizarre though - I assumed there was some proprietary component that Apple wouldn't publish the details for.

My assumption is that the development/maintenance/support costs aren’t worth the bump in adoption/sales they’d see from doing it.

It’s a large amount of expensive, ongoing work, and leveraging Apple’s native toolchain will invariably provide a better user experience.

Not if it requires me to go out and buy a machine with an OS I'm unfamiliar with. Though I gather I don't really need to interact with it directly to build Xamarin/React-native type apps (not sure about Flutter etc.)