what is my mac os operating system version

This article covers "Internet Recovery", which can download the complete "correct for this physical system" OS to a disk that is essentially unusable: https://support.apple.com/en-us/HT201314

Needs to download the entire OS (several GB) but given your disk has no usable data will help. After you have a usable OS you can recover from a backup that has Big Sur (if you have a backup) or just allow upgrade from what InternetRecovery gives you to Big Sur.

The alternate is finding another system that has been able to run Big Sur and make a "USBInstaller" of a downloaded BigSur install pack.

If you have to you can use a Windows PC to download and make bootable Mac installer. If trying what steve359 doesn't work. Though it should work. Read this for using Windows.https://pureinfotech.com/create-macos-bootable-usb-windows/

Click on the Apple Menu > About This Mac and read what operating system version you have.

You can get it from Apple here.How to get old versions of macOS - Apple Support After you click on the link you will see a link for Big Sur. click on it and it will open up in the App Store.