Why would this take a week? i(phone)OS was originally a cut-down low-resource fork of MacOS with support for touch screens and a somewhat different driver model.

It's still that, but hardware has moved on and the super-low-resource constraints have been relaxed a little.

But it still lacks many of the features available in full-fat MacOS. And there are still significant differences in the GUI APIs.

This isn't a trade secret - it's very, very obvious from the docs.