Besides the various mentions of iOS being on ARM, it's also worth mentioning iOS is a fork of macOS (née Mac OS X), and was explicitly branded as the same OS when the iPhone launched. In other words, macOS has been capable of running on ARM—at least internally at Apple—for 13 years.
It's highly likely that Apple has maintained internal macOS ARM builds for every successive release since, if not since before the Intel transition. They had similarly maintained x86 builds of macOS from the time they began its development after the NeXT acquisition.
It's highly likely that Apple has maintained internal macOS ARM builds for every successive release since, if not since before the Intel transition. They had similarly maintained x86 builds of macOS from the time they began its development after the NeXT acquisition.