How to downgrade from Sonoma to Monterey on a macmini 2018

One very Important factor missing from a previous suggestion

The computer has a T2 Chip >>

Link below

About Startup Security Utility on a Mac with the Apple T2 Security Chip

Without changing the Startup Security Setting Before your Wipe the drive.

The computer Will Not be able to Boot- Up from the Bootable Installer USB Drive

Mac computers with the Apple T2 Security Chip

First, back up data on your Mac. Then, download Monterey install app from App store and make a bootable USB with it.

After that, boot your Mac into Recovery mode and erase the disk with Disk Utility app. After that, reboot your Mac from USB and start installing Monterey.