how can I downgrade my 27" 2020 iMac from Mac OS from Catalina to High Sierra

You can't. High Sierra doesn't contain the drivers needed to boot that model.

(225774)

FWIW, the only way you can run High Sierra on a 2020 iMac is as a VM running in Parallels Desktop, VmWare Fusion, VirtualBox etc. You would still need access to a Mac old enough to download High Sierra.

Whatever Version of macOS came installed on this computer -->> when it was New ---<< is the Oldest Version of macOS it allowed to run.

The Apple Licence of Use prohibits installing Older Versions that what it came installed with Originally.

No contradiction IMHO.

As stated earlier - If the Computer Originally Came Installed, caveat here, As NEW with Catalina - the " Apple Licence of Use " Prohibits installing an Older Version of macOS.

Whether the Drivers in an Older Versions of macOS, like High Sierra are present, becomes totally inconsequential.

If going the VM Software route - this would mean there would be Two Separate Operating Systems running at the same time plus the VM Software

iMac (Retina 5K, 27-inch, 2020) - Technical Specifications and IMHO - would need more than the Stock 8 GB RAM installed in oder to keep all the Plates Spinning at the Same Time.

Your answer contradicts Niel's answer. Who is correct?