What's the best way to prevent my Mac from updating macOS?
Actually that is pretty easy to do.
I've got a 16&quot; 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 &gt; Software update and uncheck &quot;Automatically keep my Mac up to date&quot; like this:

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

Note that is is OK to leave the last option &quot;Install System data files and security updates.&quot; 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 &quot;Upgrade Now&quot; button but when there are updates for your current macOS version they will appear below that under a very small &quot;more...&quot; link. That will NOT install Big Sur. Instead it will install just the updates to your current version of macOS.