How to uninstall macOS Big sur from my 2015 MacBook Pro?

First off if you value your data you should always have frequent and regular backups. And you should always make sure you have a good backup before doing anything major to your Mac such as upgrading the OS.

Before doing anything else make sure to backup your important data. I don't know the best way to do this since you will be downgrading macOS and I don't think Time Machine will allow you access to the backup after you downgrade. Unless another contributor has a better idea I recommend you manually transfer your data to an external drive, but it is likely you will miss transferring some data and especially settings that are hidden away in the hidden "~/Library" folder. It might be a good idea to clone your drive to an external drive using Carbon Copy Cloner (CCC). After the clone completes make sure you can boot to the external drive by Option Booting.

After you confirm you can boot to the clone power off the laptop and disconnect the external drive containing the clone.

Boot normally to the internal drive again and download and create a bootable macOS USB installer for the OS you prefer using the instructions in this Apple article:

https://support.apple.com/en-us/HT201372

After successfully creating a bootable macOS USB installer you will need to Option Boot and select the USB installer. Once the installer boots launch Disk Utility. You will need to partition and format the whole physical SSD, but how you do this depends on the OS you are installing. If your laptop contains a third party SSD, then you will need to install macOS 10.13+. With macOS 10.13+ you must click "View" within Disk Utility and select "Show All Devices" before the physical drive appears on the left pane of Disk Utility. Select the physical drive which should be something like "Apple SSD ...." and erase it as GUID partition and APFS (top option).

Once the whole physical SSD has been properly partitioned & formatted (or erased if installing macOS 10.11+), then you can install macOS. If you don't do this properly you may end up with a mess of APFS volumes and error messages about incompatible file system features.

After macOS is finished installing and you are greeted with Setup Assistant you can attempt to connect your external drive containing the Big Sur clone to see if Setup Assistant will allow you to migrate just the user accounts. If Setup Assistant won't allow you to migrate the user account(s), then just create a new user account. Once you are logged into the OS you will need to manually transfer your data to the internal drive. Keep the external clone drive until you are sure or are satisfied that you have all your data (I suggest keeping the clone for several months, if not indefinitely). After you finish setting up the Mac make sure to use Time Machine to backup your Mac to another USB drive dedicated only for your Time Machine backups.

Back up your Mac with Time Machine - Apple Support