What cable do I need to connect my 2020 MacBook Air to my 2013 21.5 iMac

I'm not sure any of the 2020 Macs can do that!?

You can connect any of these older iMac models to another Mac from 2019 or earlier for use as an external display. Learn how to identify your iMac model.

• 27-inch iMac models introduced in 2009 and 2010: Use a Mini DisplayPort cable to connect your iMac to a Mini DisplayPort or Thunderbolt or Thunderbolt 2 port on the other Mac.
• All iMac models introduced in 2011, 2012, 2013, and mid 2014: Use a Thunderbolt cable to connect your iMac to a Thunderbolt or Thunderbolt 2 port on the other Mac. If the other Mac uses Thunderbolt 3, you can use the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter with your cable.

Target display mode requires macOS High Sierra or earlier on the iMac used as the external display. It does not work with macOS Mojave or later.

I read it the same way as my esteemed colleague: 2019, yes; 2020, no.

Full support article:

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

The 21.5 inch iMacs did not support Target Display Mode, so you are done before you begin.

The 2009 & 2010 iMacs were limited to 27", but All iMac models introduced in 2011, 2012, 2013, and mid 2014...