Can i link my old Mac to my new Mac for displays?

Despite what Apple says, the 27" 2009 and 2010 iMacs can be used as displays with a 2020 Mac. Connect from a Thunderbolt 3 port on the new Mac to the Mini DisplayPort on the old iMac with a USB-C to Mini DisplayPort cable.

mophie USB-C Cable with Mini DisplayPort Connector - Apple

Other iMacs cannot be used as displays with a 2020 Mac.

Use your iMac as a display with Target Display Mode - Apple Support

Not available after 2019 models.