I use Linux and I know about Window Managers and Desktop Environments and X11.
I don't want to create a desktop environment from scratch, but I want to know how WMs and DEs work together. Is a DE just a WM with an additional layer of graphics? And how do Desktop Environments work, exactly?
Finally, I want to know how people "fork" desktop environments - for example, Cinnamon forking from Gnome3. How would I do this, and what license would I use?
A DE has a WM as part of its software suite. KDE, for example, has KWin as its WM. It's generally much more than that though - it also includes things like a login manager, desktop, some sort of system control panel, basic applications like a file browser, text editor, and many more. You can use a different WM than the one that comes with the DE, or software from another DE within a different one (though you often need to have some dependent libraries).
I don't really know about forking. You'd probably clone the code, keep the previous license (I think), and find some people that can get behind your cause.