ASUS ProArt Display
The PA279CRV display is connected to MacBook Air but only wallpaper displays on monitor. How can this be fixed?
Sounds like you are running in extended desktop mode – and your MacBook Air's screen is the main screen.
If I remember correctly, Macs don't display the Dock or menu bar on all screens; just on the main one. So if you are running in extended desktop mode, the extra screens won't have anything on them until you drag windows from the main screen to them.
In Displays Settings, there should be a place to tell the Mac the rough physical arrangement of the Asus display with respect to your laptop screen. This will determine the crossing points where you can move the mouse pointer from one display to another, or drag windows from one display to the other.
Extend or mirror your Mac desktop across multiple displays - Apple Support