Advanced users can use the command softwareupdate --fetch-full-installer --full-installer-version 11.1 . Key Features of Version 11.1 (20C69)
Apple provides all macOS operating system updates at no cost. If you see an advertisement or a third-party site asking for payment for this specific download, it is likely a scam or a "pirated" distribution that could contain malware.
Search for "macOS Big Sur" directly in the Mac App Store .