What's the best way to prevent my Mac from updating macOS?

Actually that is pretty easy to do.

I've got a 16" MBP on which I do not want to upgrade to Big Sur (at least yet) and here is what I have done.

First go to System Preferences > Software update and uncheck "Automatically keep my Mac up to date" like this:

Then click the "Advanced..." button and un check everything except the first option, like this:

Note that is is OK to leave the last option "Install System data files and security updates." But being the mildly paranoid techie I choose to leave that off. Your call tho.

Note that there is really no way to hide the Big Sur "Upgrade Now" button but when there are updates for your current macOS version they will appear below that under a very small "more..." link. That will NOT install Big Sur. Instead it will install just the updates to your current version of macOS.