Where Apple says something is an OS, they really mean it as a distinct user experience. They want to make the infrastructure that applications rely on as simil

Where Apple says something is an OS, they really mean it as a distinct user experience. They want to make the infrastructure that applications rely on as similar as possible, including UI efforts like Catalyst and SwiftUI. But at the same time, they know that the user interface on a iPhone is fundamentally different than what is shown on a TV. They added Xbox/dualshock controller support specifically for tvOS - but wound up adding it to iPhone and iPad since they were really adding to the underlying platform.