2017 iMac as display for 2012 iMac

You cannot.

Read carefully the Related Article linked above your post:

Only the following older iMac models could be used as an external display when plugged into another older Mac model:

• iMac models introduced in 2011, 2012, 2013, and mid 2014 using Thunderbolt or Thunderbolt 2 cable.
• 24-inch and 27-inch iMac models introduced in 2009 and 2010 using Mini DisplayPort cable.

There is potential for you to use the 2012 iMac as a display in this way for the 2017 iMac, given the following caveats:

• The intended target iMac must be running macOS 10.13 High Sierra or earlier; it does not work with macOS 10.14 Mojave or later, or with Boot Camp and Windows.
• Retina display iMacs won’t work as target displays.
• In all cases the source Mac that you're connecting FROM must have been introduced in 2019 or earlier and have macOS 10.15 Catalina or earlier installed.

There is a software option called Luna Display

What OS' do you have on both machines?

The article linked above is pretty specific as to what will work.

The only way I have found is a capture card or using Airplay, and yes. I am running Ventura on my 2013 iMac with OCLP and it does support airplay and it works beautifully