Is it possible to install the 11.2.1 update OVER an existing 11.2.1 system? That used to be possible when combo updates were available (RIP combos). There is a Big Sur download available at App Store but I don't know if it's the whole shebang or just the update. (I suspect it's the entire OS.)
As there is only one downloadable version of macOS Big Sur available on the App Store it has to be a complete version of macOS 11.x, or else it couldn't be used to upgrade from Catalina or earlier macOS versions. In fact, if you follow
these instructions you can use it to create a bootable USB installer on a thumb drive. It works great. There is your combo updater too! Unfortunately if you should want to downgrade or think you might, be sure and keep a copy of each version from the App Store.
