After USB-C win, EU tells Tim Cook that Apple must ‘open up its gates to competitors’.::The iPhone 15 has USB-C, a move largely due to impending legislation in the European Union requiring smartphones and other…
In principle you can, the Mach-O format is openly documented and implemented in the major compilers. The issue is that you need a sysroot (aka SDK) of the frameworks and headers for your target OS, which in Apple’s case are proprietary and cannot be redistributed legally (you could probably rip them out of a macOS installation yourself though). For iOS apps you’d also need to sign the binaries and install the app to the device which is non-trivial to impossible to do on other platforms.