Alan Kay did actually say that he wished he'd built Smalltalk's GUI around a single class: "a smart rectangle that can contain other smart rectangles". In other words, Windows windows.
So like X11 windows when using the original drawing API, buttons and other elements are just a window inside a window that receive events when they have focus.
Yes. One of the reasons why Wine is a thing is because the primitives provided by X11 map nicely onto those provided by Windows. No Wayland port of Wine is in the immediate offing, also for this reason.