The original way of doing they was just to turn off Mirror displays. That gave you one Extended desktop (including the built-in display), onto which you could move whatever windows you wanted. Every Item in this panel is an Icon, and ALL can be re-arranged. Once you get the displays in proper orientation, the mouse moves across the break between displays in a continuous motion, and can drag a window with it. A window parked on the break between displays is updated "the right way" when you scroll:

later, "displays have separate spaces" was added, in case you want to use a particular display for only certain information with no windows overlapping or splitting screens.