What I mean is that iOS is a fork (or, I suspect, maybe just a build configuration) of macOS. Since it was a new platform, Apple could try some new things and e

What I mean is that iOS is a fork (or, I suspect, maybe just a build configuration) of macOS. Since it was a new platform, Apple could try some new things and enforce new restrictions (like sandboxing), but they're not completely different codebases in the way that Windows CE and Windows NT are. The core technologies are the same at most levels of the OS and, if they're not maintained a single codebase, clearly many components are. Therefore I strongly suspect that Apple had macOS running on the iPhone and iPad from very early on. They just did not want to release it because the UI is not suited to a phone.