How to downgrade my macOS version?
I tried to install Big Sur to my Mac computer and something went wrong. How do I get my computer back to the way it was when I bought it?
[Re-Titled by Moderator]
Perform a clean install of macOS by booting into Internet Recovery Mode using Command + Option + Shift + R to boot into the oldest online macOS for your Mac (usually for the OS that shipped with the Mac from the factory). If you have access to another compatible Mac which can run the previously installed version of macOS, then you can create a bootable macOS USB installer.
You will need to erase the whole physical drive, but how you do this depends on which version of macOS you end up installing. If you are booting a macOS 10.13+ installer, then you need to click "View" within Disk Utility and select "Show All Devices" so that the physical drive appears on the left pane of Disk Utility. Then erase the physical drive as GUID partition and APFS (top option) if installing macOS 10.13+, otherwise erase the physical drive as GUID partition and MacOS Extended (Journaled) if installing macOS 10.11 or 10.12. If you are installing macOS 10.6 to 10.10, then you need to partition and format the physical drive using the instructions in this article:
https://www.owcdigital.com/assets/support/support-formatting-and-migration/Mac_Formatting_6-10.pdf
Keep in mind a clean install erases & destroys all data on the drive so I hope you have your data backed up.
You should always have frequent & regular backups.