UIWindow and addsubview method

Every video tutorial and book I have read displays the following code to add a UIView to the UIWindow.

[window addSubview:self.viewController.view];

My understanding of the above code is that a "View" (which is an instance of a UIView) is added to the window (Which is an instance of UIWindow). Let me break it down (According to my understanding):

window (UIWindow) addSubview (method to add a View to a window) Self.viewController.view (simply returns an instance of a "view" which is already instantiated within the UIViewController class.

The first problem I have is that I could not find the method "addSubview" in the UIWindow class reference document on apples site. However somebody kindly pointed out to me that UIWindow inherits addsubview method from UIView. thats all fine, but why do all the book and online documents state that the addsubview method adds a view to the window - but how can that be? really confused. Can somebody please explain step by step what this code is doing? If UIWindow inherits the addsubview method of UIView then how can it go back up the inheritance tree? really lost. What I really need is small example code with diagrams of what is happening step by step. would be REALLY greatfull. many thanks