In some atomic distributions like Vanilla OS and SteamOS you have two mostly (AFAIK) identical root partitions. Let’s say that you boot partition A. You install a system update, and now partition B contains the updated OS. Your bootloader also remembers which partition is up-to-date and boots off of that one next time. Now, you boot with partition B. There is a system update and the latest version of the OS is installed to partition A. It now repeats as you might expect.

The goal is to have both versions readily available so that if an update introduces problems you can always continue to use the old one. This is not something you are meant to think about or tinker with. If you need more customization, I think you might wanna consider a more traditional distro.