Where do i download THIS version??

Agree with @ MartinR regarding the Compatibility Issue

link below

macOS Big Sur 11 hardware requirements

If the User whats an older version of Big Sur and not the Most Up to date version

In Terminal run this command which will Only List all available Full Installers this computer Qualifies to run.

softwareupdate --list-full-installers

Example of what may appear

macOS Big Sur, Version: 11.7.2, Size:

* Title: macOS Big Sur, Version: 11.7.1,

* Title: macOS Big Sur, Version: 11.7,

* Title: macOS Big Sur, Version: 11.6.6,

* Title: macOS Big Sur, Version: 11.6.5,

* Title: macOS Big Sur, Version: 11.6.4,

* Title: macOS Big Sur, Version: 11.6.3,

* Title: macOS Big Sur, Version: 11.6.2,

* Title: macOS Big Sur, Version: 11.6.1,

* Title: macOS Big Sur, Version: 11.5.2

If the user posts back a reply with that Exact Version that are looking for we can assist in that request.

First, your Mac needs to be compatible with Big Sur -> macOS Big Sur is compatible with these computers - Apple Support

Then -> How to download macOS - Apple Support

Use Safari to download macOS.