Connect your pen display to the computer and refer to the following display modes to set an additional monitor on Mac.
Mirror Mode. What is shown on your computer will be duplicated to the pen display; called Duplicate display on Windows.
Extend mode. Your desktop will be extended and displayed in two separate monitors, with each one showing different parts of the interface.
A. To Mirror your Display in MacOS
Mirror display is 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 which, you will be in the Extend mode, where the display with a white bar indicates the primary display.
As shown below, the small screen is your MacBook, while the large one is the pen display. To change your primary display, just drag the white bar to the other display.
You can also rearrange the displays and relocate the menu bar as you desire.