"Extended desktop" is just the word to use to talk about external monitors, the physical layout doesn't matter.
When you plug in an additional monitor to your Mac, you have the option of mirroring the contents of the Mac's display to the external or you can use the external as an extended display that's separate from the Mac's display.
Since my Mac Studio doesn't have a built-in display, I have three options. I can choose either of my monitors the the main, either of them to be extended, or I mirror the two of them (which just seems silly).
Here are settings for my first screen.
And here are settings for my second screen.
Not that one of them says
Main Display and the other is
Extended Display
In my case, I have the two monitors side-by-side and right monitor is the main display and left is extended. I also have the Dock on the right side of the main display..