To get Big Sur try first entering this command in Terminal to find what macOS versions are available:
softwareupdate --list-full-installers
I got this a couple of days ago:
Title: macOS Monterey, Version: 12.0.1, Size: 12128428704K, Build: 21A559
* Title: macOS Big Sur, Version: 11.6.1, Size: 12428472512K, Build: 20G224
* Title: macOS Big Sur, Version: 11.6, Size: 12428553042K, Build: 20G165
* Title: macOS Big Sur, Version: 11.5.2, Size: 12440916552K, Build: 20G95
* Title: macOS Catalina, Version: 10.15.7, Size: 8248985973K, Build: 19H15
* Title: macOS Catalina, Version: 10.15.7, Size: 8248854894K, Build: 19H2
* Title: macOS Catalina, Version: 10.15.6, Size: 8248781171K, Build: 19G2021
* Title: macOS Mojave, Version: 10.14.6, Size: 6038419486K, Build: 18G103
* Title: macOS High Sierra, Version: 10.13.6, Size: 5221689433K, Build: 17G66
Then enter the command softwareupdate --fetch-full-installer --full-installer-version xx.x.x
where xx.x.x is the version number in the list you wanted