The Switch Monitor UI is kind of useless and broken on the desktop
The Switch Monitor UI (which is bound to Windows-P by default) is kind of useless and broken on the desktop.
"Join Displays" and "Mirror" make sense and work correctly, but "External Only" and "Built-In Only" are nonsense -- this is a desktop PC, not a laptop or an all-in-one, it doesn't have any built-in displays, all of them are external. Selecting either of "External Only" or "Built-In Only" results in a "Joined Displays" layout, which is neither useful nor what I'd expect from the labeling.
Instead of the "Built-In Only" or "External Only" options, the Switch Monitors widget should display a list of active connectors labeled both with the name of the output (e.g. DVI-D-0) and the vendor/product name of the attached monitor (e.g. Dell Inc. 23").