DM, WM?
On every Linux I've ever used the dm is the display manager, XDM, KDM, GDM, and this is what the desktop environment rides on.
Then the desktop itself IS a "window manager," though the only one still using that in the name is IceWM. But the "K desktop environment" is a window manager. The "Gnome desktop environment" is a window manager. XFCE etc are all window managers.
On every deskop I've set up with Linux (there's been hundreds) the service "dm" starts at boot. But you do not land on a desktop. You only get to a log in screen. After authenticating the user's window manager of choice is then loaded. That's what yields any "desktop environment." (doesn't have to tho)
That last is a matter of semantics, "desktop environment" sounds like marketing. I've always known it as the window manager, and some of them that's barely all they do.
But certainly under it all is the "display manager," which is more concerned with drivers and hardware than the WM is.
