How do I use a MacBook as an external monitor?

I'm not sure that's possible. I mean, you can use an separate external monitor with your MacBook Air or with your other kinds of Macs, providing you have the right adapters, in closed display mode, and you can use certain iPads as an external monitor utilizing Apple's SideCar feature, but I am not certain that you can use one laptop, open, as an external monitor for a second open laptop.

But if it can be done, someone on here has no doubt done it and they'll swoop in for sure. :-) Otherwise, if you have a SIdeCar compatible iPad around, maybe that might work.

-- JDee

This is not a feature and never has been—mac as an external monitor.

Follow the sidecar advice above with a compatible iPad.

Get a monitor. You can use it as a second display for either Mac.