I'm kind of curious about what layer writing a UI platform to the level of Win32 or the X Windowing System would fall in the grand scheme of an operating system. What layers below do they primarily make use of, is it heavily based on direct communication with the graphics card driver (I can't imagine going though a rendering pipeline like OpenGL for this), or is there a graphical platform as part of the operating system which extracts this out a little more.

I'm also interested in the creation of shells and the like, and I"m particularly curious as to how people go about creating alternative shells for windows, what do people look for when figuring out what methods to call or what to hook into, etc? I guess I'm fairly lost at these concepts and finding it difficult to find documentation on them. I was initially excited to have taken Operating Systems in college but it was all low level resource management stuff.

Thanks all, Mason