iPad or Mac as a second screen

Starting with macOS 10.15 Catalina, Apple integrated Sidecar, which lets you use an iPad as a second screen for a Mac. Apple did not invent the idea, however: third-party solutions had already existed for several years.

Duet Display or Air Display let you use an iPad as an additional screen. You install an app on the iPad, usually paid, and its equivalent on the Mac.

Another solution is Luna Display. It requires a small hardware adapter plugged into the main Mac, Mini DisplayPort, Thunderbolt or USB-C depending on the model.

The advantage of Luna Display is that it is not limited to the iPad. It can also use another Mac as a second screen, for example turning the screen of an older iMac into an external display for a MacBook or Mac mini.