Use extended desktop mode:
① Choose Apple menu > System Preferences, then click Displays.
② Click the Arrangement tab.
③ Make sure that the Mirror Displays checkbox isn’t selected.
④ Arrange your displays to match the setup on your desk. To change the position of a display, drag it to the desired position. A red border appears around the display as it's moved.
⑤ To set a different display as the primary display, drag the menu bar to the other display. The primary display is where your desktop icons and app windows first appear.
Mirror your displays:
① Make sure that your external display is turned on and connected to your Mac.
② Choose Apple menu > System Preferences, click Displays, then click the Arrangement tab.
③ Make sure that the Mirror Displays checkbox is selected.
