2020 iMacs do not support Target Disk Model. Apple started TDM's deprecation in 2014 and threw the last shovel of dirt in its face in early 2020.
As dialabrain mentions, at least one person here has managed to create an unsupported pairing of a 2020 Intel-powered Mac with a 2009 or 2010 iMac 27, but there are an estimated 3 million users here and other cannot make it work. The 2009 and 2010 iMac had different ports than newer iMacs and were always more forgiving of the video source. Some even worked (unsupported by Apple) with gaming consoles and Windows computer, but that was 11 years ago.
In general, it's dead. You could spend US$60-90 on cabling and still not have a working setup. I can get a nice external monitor for US$90!
The current hardware and system requirements for TDM are:
VIDEO SOURCE COMPUTER (the one sending the video signal to an external monitor):
- A 2019 or older Intel Mac running macOS 10.15 Catalina or older.
- Apple Silicon Macs are not supported
- mac OS11 Big Sur is not supported.
TARGET COMPUTER (the one to be an external monitor):
- An iMac 27-inch (Late 2009 and Mid 2010)
- any iMac from Mid 2011 to Mid 2014.
- The target computer must run no higher macOS than OS 10.13.X High Sierra.
