Using MacBook Air with iMac as an External Monitor

Hi there:

I have a Macbook Air (M1, 2020) and I have an iMac (21.5" 2017). I'd like to use the iMac as an external monitor when I want a bigger screen. I'm struggling to use a thunder cord or even a USB-C to connect the two computers as it appears as if they don't want to talk to one another. Is there a way to do this or are they incompatable? Can use screen share without connecting any wires (like a sidecar or airplay)? Someone lend me a hand and let me know how to do this, please.