It depends on the type of port your monitor has. You need a USB-C to HDMI adapter cable, USB-C to DisplayPort cable or USB-C to USB-C cable. (I’m not sure if anyone has tested USB-C to VGA or DVI so far, but that probably works as well.)
See: Frequently Asked Questions · Wiki · Librem5 / Librem 5 Community Wiki · GitLab
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