Use a 2017 iMac Retina as an external monitor for a 2020 Macbook Air M1

As my main Mac I have a 2020 Macbook Air M1, OS 11.6. I also have a 2017 iMac Retina 21.5", OS 11.6 also. After researching some on this question, I called AppleCare and she said I could not connect the two with a cable to have the 2017 act as an external monitor for the M1. However, I have been able to connect this M1 to my TV with an HDMI cable and an HDMI adaptor plugged into the USB port on the M!. This lets me then use settings to make the various selections so that, as example, I can have one program running on the TV screen, and a different program running on the M!, and using the M1 trackpad, I can move the cursor across from one display to the other. I thought I'd be able to do the same using the 2017 instead of the TV. The advisor said this can't be done and sent me an article "Use your iMac as a display with target display," which seems to indicate the some old models of iMac are/were able to do this with Thunderbolt or Thunderbot 2 cables, or in the case of 2099 and 2010 models, with a mini display port cables.

I find this puzzling: that a nearly brand new M1 and a not too old 2017 cannot do what some older models could do, and can be done with an older TV with HDMI. Are there no adaptors out there to do this? There may be - I just can't find one. This photo, although upside down, shows the ports on the 2017 imac. Thise 2 thunderbolt are same size as the port on the M1.

Thanks for any advice.