How do I stop Big Sur from installing on my computer?
Disable the Automatically keep my Mac up to date check box.

Use the second “more info ...” blue link to get security updates until you are ready to take the invitation to upgrade. It’s listed below the Other Updates text in the middle of the window.  If you like the command line you can pick and choose from any or all available update(s) and also suppress them by name.
sudo /usr/sbin/softwareupdate --ignore &quot;macOS Big Sur&quot;

Please read this entire article before trying sudo for the first time, you could delete all your documents and photos with a syntax error or bad copy/paste of a good command. If you use Time Machine, one sudo alone won’t delete your backups.

https://derflounder.wordpress.com/2020/11/12/preventing-the-macos-big-sur-upgrade-advertisement-from-appearing-in-the-software-update-preference-pane-on-macos-catalina/

The article above explains the down side of trying to block Big Sur before it's advertised and the error message you will see in that case. A more invasive solution would be to use an MDM product to block the actual installer. Or compile an app like the following open-source tool which anyone can compile with a free AppleID and free version of Xcode.

https://github.com/hjuutilainen/bigsurblocker