iMac as external display

I have a 2021 imac with macOSMonterey 12.0.1 and a 2019 macbook pro with macOS Big Sur. I would like to use my imac as an external monitor for the laptop. I have tried connecting the two with an HDMI cable which did not work. Then I tried connecting them with a thunderbolt 3 cable which did not work either. I do not understand why these $1,000 computers are such a pain in the *** to connect. I am tired of spending money on cables. Someone for the love of g*d please tell me how to get this to work.