OP:
To "go back" to Big Sur, you're going to have to COMPLETELY ERASE the internal drive and start over. So, you must BACK UP YOUR DATA first, or it will be gone.***
I'm thinking that you can't use internet recovery to do the install, because once you erase the drive, IR is going to "offer you" Monterey instead of Big Sur.
So...
You'll probably have to create a bootable USB flash drive installer.
You'll need a flash drive that is 16gb or larger.
You'll need a copy of the Big Sur installer (as mentioned above).
I suggest you use one of these small, free apps to create the bootable installer:
- DiskMaker X
- Install Disk Creator
Once you have the flash drive installer, do this:
1. Boot from the installer (hold down the option key at boot to bring up the startup manager).
2. Open disk utility
3. IMPORTANT: go to the view menu and choose "show all devices".
4. Look at the list on the left. The topmost line for the internal drive is the "physical drive" itself.
5. Erase the drive to APFS, GUID partition format.
6. Now quit disk utility, open the installer app
7. The Macbook will reboot one or more times, and the screen may go dark for a few minutes with no other indication of activity -- be patient.
8. When done, you should see the initial setup screen (choose your language).
9. Begin clicking through. Setup assistant will ask if you wish to migrate from another drive. At this point, you can TRY a time machine backup, but I have never used tm so I don't know if it will permit a migration from "a newer OS".
(for this reason, I'd suggest downloading either CarbonCopyCloner or SuperDuper and create a cloned backup BEFORE you boot to internet recovery).
Sound like a lot of work?
It is.
But if you want to "get back, get back, get back to where you once belonged", that's what you have to do...
*** Before going further, WHAT YEAR is the MacBook Pro, and what version of the OS originally shipped with it?
IF the original OS was in fact Big Sur, you MIGHT be able to coax internet recovery into installing it directly using this key combo at boot:
Command-SHIFT-OPTION-R
This is a special version of internet recovery that will attempt to install the OS that the Mac originally shipped with.
IF the installer will offer you this, you can skip having to make a bootable flash drive, and do the install "directly".