As has been mentioned, Apple does not allow downgraded on iOS devices. There's nothing to stop you from downgraded a Mac back to the version of macOS it shipped with, or any newer release it supports. And it requires no magical or unsupported steps to do it.
Our 2018 mini and our M2 Pro mini are currently running Sonoma. But if I wanted to, I could downgrade the 2018 back to Mojave (or any version above that), and the M2 Pro to Ventura. I could also have multiple OS versions installed on the same Mac.
a) Download whatever version of Mac you want that is supported by your Mac.
Create a bootable installer for macOS - Apple Support
b) Get a flash drive large enough to hold the OS and use the Terminal command on the same page linked to above to create a bootable installer.
c) Plug the USB installer into your Mac. For an Intel based Mac, restart and hold down the Option key. Choose the flash drive as the startup volume. For an Apple Silicon based Mac, power the Mac down. Press and hold the power key until you get a screen to choose a startup drive. Choose the flash drive.
d) Once the Mac has finished booting to the flash drive, run Disk Utility and erase the main drive.
e) Install the OS you loaded onto the bootable USB drive.
Rather than erasing the drive, you could also use Disk Utility to create a new volume or hard partition to install the older OS to. Then you'd have a Mac with multiple OS installs to choose from.
Is there any risk? Yes, but that's just normal mistake of not backing up your data to a removable drive before doing any of this so you don't lose it if anything goes wrong. Personally, I have done variations of this on multiple Macs over many years and have never lost any data. But it's always better to be safe than sorry.
