MacOSDisplayModes

To start, open your System Preferences from the Dock or Applications folder. Once you have opened it, click Displays.

Click the Arrangement button to adjust display position and which display is the main display. Alternatively, you can use the "Use as" dropdown in the Display Settings to adjust which display is the main display.

By default, the displays are used in extended mode. If you wish to switch to mirror mode, select the display you want to use in mirror mode and under the "Use as" dropdown, select the "Mirror for Built-in Display" option.