Connect your pen display to the computer and refer to the following display modes for settings of an additional monitor on Mac.

Mirror Mode. What is shown on your computer will be duplicated on the pen display, which is called 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.

To mirror your display in macOS.

Mirror display is default on macOS as 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.

To set Extend Displays on macOS.

Choose Apple menu > System Preferences > click Displays and then select the Arrangement tab to unselect the Mirror Displays checkbox, and thereafter you will enter the Extend mode, where the display with a white bar is the primary display.

As is shown below, the small screen is your MacBook, while the large one is the pen display. To change your primary display, you will need to drag the white bar to another display.

You can also rearrange the displays and relocate the menu as you desire.