Connect your pen display to the computer and refer to the following display modes to set up an additional monitor on a Mac.
Mirror Mode. What is shown on your computer will be duplicated on the pen display. Referred to as "Duplicate" display on Windows.
Extend mode. Your desktop will be extended and displayed on two separate monitors, with each one showing different parts of the interface.
A. To Mirror your Display in MacOS
Mirror display is the default in MacOS when your pen display is connected.
Choose Apple menu > System Preferences > click Displays, and then select the Arrangement tab to make sure the Mirror Displays checkbox is selected.
B. To Set Extended Displays on MacOS
Choose Apple menu > System Preferences > click Displays, and then select the Arrangement tab to deselect the Mirror Displays checkbox. After that, you will be in Extend mode, where the display shows a white bar indicating the primary display.
As shown below, the small screen is your Mac display, while the large one is the pen display. To change your primary display, drag the white bar to the other display.
You can also rearrange the displays and relocate the menu bar as you desire.
If your mouse cursor only works on your computer screen after extending your display (and not on your tablet), change your tablet driver's work area settings by following the steps in this article: How to Set Your Tablet Display Work Area.
For instructions on a specific MacOS version, please refer to this link: Extend or mirror your Mac desktop across multiple displays.