Video mirroring shows the entire desktop on each display.

Note: You must be connected to a display to see these options.

1. On your Mac, choose Apple menu  > System Settings, then click Displays in the sidebar ( You may need to scroll down. )
2. Click the name of your display, and then select Mirror for Built-in Display

To quickly mirror your Mac display to an external monitor, press Command + F1. Pressing Command + F1 again will revert the display to its original orientation.