2007-27" or 2013-27" as 2nd monitors with 2020 27": doable?

[fhopper]

I have the old big dog 2007 and the big dog 2013 (2020 replacing 2013) I have never used a thunderbolt port for anything but I understand the specs have changed through the iterations. Would I need some kind of dock with various versions of Thunderbolt? Is there a single cable that will work for either the 07 0r 13? Everything I read just says "use a thunderbolt cable and boot the 2nd into target mode". There has to be more to it with different generations of ports... Help me understand please. BTW I look forward to a lower resolution 2nd screen so the desktop appears as larger print. I have used an old Samsung SyncMaster 19" 1024-1280 in Portrait mode for years and the larger image size is nice.

 

[DeltaMac]

2007 iMac would be a 24-inch, too old for anything Thunderbolt, and not capable of target display.
Guessing you mean a 2009 27-inch. That would still not have Thunderbolt, and target display on that one only works with another Mac with a mini DisplayPort.
Some app that does screen sharing might work OK for you, but no direct connection.

 

[fhopper]

2007 iMac would be a 24-inch, too old for anything Thunderbolt, and not capable of target display.
Guessing you mean a 2009 27-inch. That would still not have Thunderbolt, and target display on that one only works with another Mac with a mini DisplayPort.
Some app that does screen sharing might work OK for you, but no direct connection.

It has been so long since I gave that to a daughter I had forgotten it was a 24". So that leaves the late 2013. Will it hook up as a second monitor?

 

[DeltaMac]

Late 2013 iMac should be OK as a second display for your 2020 iMac. You just need the right Thunderbolt adapters.
more info:

Use your iMac as a display with target display mode - Apple Support

Target display mode is a feature available with certain older Mac models.

support.apple.com