iOS, tvOS, and watchOS share a great deal with macOS (past the surface, they’re almost identical). They’ve probably had an ARM build of macOS running internally

iOS, tvOS, and watchOS share a great deal with macOS (past the surface, they’re almost identical). They’ve probably had an ARM build of macOS running internally for years now, much as they had an x86 version of macOS years before the PowerPC -> Intel transition. The transition will be harder for third party devs, but there having been popular ARM Apple platforms for over a decade will speed things up considerably. Many Mac apps will have ARM versions available overnight.