Earlier quoted context omitted.
You sure you are not confusing it with iOS? That's where you always have strictly only one app in fullscreen! (Yes, yes. It is a lame joke. But who uses only one Window at a time? What is the point of that? Though I remember when I used OS X you could swipe left (or right) to switch back to all other apps. So I think that was good enough.)
I use several windows at a time (around 5 in average), but they are all maximized. I just alt-tab and switch workspaces?
In the olden days, Mac users would tend to have lots of overlapping windows. Dragging anywhere on the window edges would move the window, so it was easy to arrange them as you wanted - almost like shuffling bits of paper on your desktop (strangely enough) and if you wanted to move something out of the way, you could fold it up (window shade). There was no need for maximising or minimising and "zooming" just meant "make this as large as makes sense for this particular document", not "expand to take up all the space on my desktop"
As OSX/macOS has developed, all that document/desktop-style behaviour has been lost.