I was working on emulating apps directly [1] by translating API calls from iOS to Windows APIs using WinObjC [2]. Unfortunately, WinObjC got abandoned and didn't even contain as many APIs as I thought, so the result cannot emulate complex apps. But it was fun - lots of low-level stuff, patching 3rd-party code, etc.
[1] https://github.com/ipasimulator/ipasim
[2] https://github.com/Microsoft/WinObjC