Whenever you run `nixos-rebuild switch` a symlink is created for `/run/current-system` which is the current system. That's what is used on every boot until anot

Whenever you run `nixos-rebuild switch` a symlink is created for `/run/current-system` which is the current system. That's what is used on every boot until another change (you also have an option in boot loader to boot into prior versions (it is just a symlink))