When you connect your Mac laptop to a projector (or an external monitor), you have two options to configure the displays. You can choose which one you want to use from the Displays System Preference pane
Once the projector is powered on and connected, you can select which option you want to use from the Arrangement tab on the (the Arrangement tab will only be visible once the external projector is On and connected)
Option 1: Mirror Displays
If you enable that option, your laptop screen and the external display will show the same content
Option 2: Extended desktop
When you disable Mirror Displays, both the laptop's and external screens will act as a single, continuous desktop where you can decide which windows will be displayed on each of those screens. That can be useful when you want to work on something on your laptop screen while showing a document or a movie in the projector.
By dragging and dropping them, you can place the displays in a position that matches their real world ones so it's easier to move the mouse and windows from one screen to the other.