Using iMac Late 2015 as a second display screen for MacBook Air 2017
How can I use my iMac 21.5inch late 2015 with Monterey 12.4 OS as a display for my MacBook Air 13 inch 2017 with Big Sur 11.2.3 OS? I've tried a thunderbolt cable and the Fn-Command-F option, neither of those work. Screen sharing doesn't work either.
My iMac has become so slow that I thought I'd use my MacBook instead, but I like the big screen of my iMac. Anybody got any ideas?
iPad Pro, iPadOS 16