You can also trick any HDMI output to believe it's connected to a monitor.

One commercial product is:

https://eshop.macsales.com/item/NewerTech/ADP4KHEAD/

But I seem to recall there are dirt cheap hacks to do same. I may be conflating it with "resister jammed into DVI port" which worked back in the VGA and DVI days. Memory unlocked - did this to an old Mac Mini in a closet for some reason.