Usually plugging an external screen into the HDMI port, or using an adapter from a USBC port, will allow you to use the computer from that screen. Sometimes there is an issue where the external screen doesn't display anything, or the login window is on the non-working screen.
To force the external screen to be used, do this...
Shut down the computer. Plug in the external screen, and a keyboard and mouse. Press the power button and close the screen as soon as the chime starts. The laptop should go into "clamshell mode" using just the external screen.
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