ubuntu workspaces with extra monitors

I'm new to Ubuntu and I really don't know how workspaces work. I'm using a laptop hooked up to a desktop monitor. My laptop runs Ubuntu 17.10

I have the gnome tweak tool installed. (I think. There's some kind of app that's just called "Tweaks" and I'm not too sure.)

I see several options under the workspaces tab, but I'm not too sure what some of them do.

There's one called "Workspaces on Primary Display Only. Additional displays are treated as independent workspaces".

• Does that mean my primary display, say my laptop monitor, is the only one that can use workspaces? Or does it mean that my laptop monitor and my desktop monitor are different workspaces?
• Can my laptop and desktop access the same workspaces? Say I was using my laptop separately with 4 workspaces, then decided to connect the desktop monitor. Can I view my current workspaces on the desktop monitor? How do I do that? The usual keyboard shortcuts don't seem to work when I have my mouse on the desktop monitor.

"Workspaces Span Displays. The current workspace includes additional displays."

• It seems that regardless of whether I have this option checked or not, I can still drag windows between my laptop and desktop monitor. When I change workspaces, whatever I have open on the desktop monitor also disappears, as if it were just an extension of the workspace I was using previously with the laptop. What's the point of this option?

What I want to be able to do is keep the 4 workspaces I usually use, and simply use the desktop monitor to view a different workspace than the one on my laptop. I want to be able to freely set any of my monitors to view any of my workspaces. How do I do this?