How can I retro downgrade my system to an easier OS system?

Understand downgrading the computer can only be downgraded to the Version of macOS it came installed with as New.

Example: computer came preinstalled with Big Sur - that is as far back as it can be downgraded to and nothing Older.

The " Apple Licence of Use Agreement " prohibits downgrading any further.

Should this Not be the case >>

Understand - the below will WIPE ALL DATA and there is No Recovery - Period

To perform this action will require booting from a Bootable Installer. 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer. Example : Bootable Installer of Big Sur would have to be done on a computer that Qualifies to run Big Sur.

Once that is done read on for preparing the Destination computer  >> Only works on Intel Based Apple Computers.

Extra Special Notation regarding the Touch ID equipped Apple Computer.

About Startup Security Utility and Must Enable from Recovery Mode the ability to boot from External Drive Before Attempting 

1 - Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

2- Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

3 - It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

4 - Choose the Upper Most Drive ( not the volumes indented and list below ).

5 - The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map. This applies to macOS 10.14 Mojave and above. 

5A - Formatting for macOS 10.13 High Sierra and below requires HFS Journaled with the GUID Partition Map

6 - Once that is done >> backup out of Disk Utilities and choose install macOS. 

7 - Follow the prompts and it may automatically reboot several time. 

8 - Upon a final reboot - Setup Assist will present with the newer version of macOS.

Hi,

See: How to get old versions of macOS - Apple Support

That link will only take you as far down as El Capitan, which is newer than Yosemite. If you want to get something like Mavericks or earlier, you might have to find an installer online which is risky. Don’t get one from weird, shady looking websites. If you have grey or RTM install discs for Mountain Lion and earlier then you can just use those.