downgrading from Monterey back to Big Sur

Correct, any Mac newer than than October 24, 2021 in its own release date of the specs of the Mac, can't downgrade to Big Sur.

Those that can downgrade can use internet restore to erase or partition and install a separate installation of Big Sur or earlier.

Internet restore is described here:

https://support.apple.com/guide/apple-configurator-2/revive-or-restore-a-mac-with-apple-silicon-apdd5f3c75ad/mac

on Apple Silicon, and is command-option-shift-R on Intel Macs.

See this article:

https://discussions.apple.com/docs/DOC-1948

The iMac 24" came new with what version of macOS ?

That will dictate what version it can be downgraded to.

If it came New with Monterey - there is no downgrade to Big Sur as per Licence of Use as er Apple.

You erase the drive and restore from the Time Machine Backup you made prior to installing Monterey.

Also see:

How to downgrade to Big Sur?