Linux GUI stack confusion [closed]

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I am trying to understand how Linux GUI stack works. Let me explain:

In Windows, thigs are relativelly simple. You have GDI/GDI+. It handles all the subsequent operations from windows drawing and positiong to drawing buttons etc, right?

But in Linux I get pretty confused. Maybe you better understand where my confusion comes from if I explain my thoughts. So, first of Linux I read about its desktop managers. Gnome and KDE. So I picked KDE (for no specific reason) and learned it is based using Qt library. So I read about Qt library a bit more.

I first thought that Qt actually renders UI elements like buttons, sliders and so on. But when I saw example for Windows since its multiplatform, I realised it does not. It is using GDI for rendering. So the Linux version must use some Linux way to render UI elements.

So if I am right, KDE uses Qt just to organize things, I would say in very simplistic way as layout manager, right? I assume this, if on Windows is it using GDI for rendering, its widely used just becouse its simpler and cleaner then directly manipulating GDI.

So from this point of view, Linux desktop (actually windows too) is "just" a window which is always fullscreen and cannot be minimised, shut down and so. It is using Qt for higher level of rendering basic UI elements. But that means there is another deeper layer under Qt library. I read about X system and its window managers. Are X window managers the layer that renders UI elements (buttons and so on) ? Becouse if I am right, X system is "just" a graphical interface between upper levels and graphical subsystem of PC. Something like GDI use DirectDraw to access framebuffer etc...

In Windows this whole stack seems more compact, I am NOT saying it is better, becouse GDI seems to be in role of Window manager and UI elements renderer together. I believe this is why advanced UI interfaces (Compiz...) are developed for Linux.

So, please, where am I wrong? I tried to understand it as much as I could, but I still think I miss something. Thanks.