External display using and old iMac
Can I use an old iMac (20 inch, Mid 2007) as an external display with MacBook Air (13 inch, 2017) and if so, how?
Question marked as
Please refer to Use your iMac as a display with target display mode. It has been archived, so its information may no longer be applicable.
There are hardware and software requirements: "Make sure that your iMac is using macOS High Sierra 10.13.6 or earlier. You can't use target display mode with later versions of macOS, or with Boot Camp and Windows."