Yep exactly, the entire server is defined with one file and then the system becomes immutable. Which really just means the only way to jack up your installation is to mess with said config file. It won't rebuild your system if there's any errors as well which is a bonus.
Discussion
Is this just for servers?
It's mainly for servers but you can also use it on desktops. It's actually the only distro I use anymore, from my pi's to servers to desktop and laptop. All of em are on nixos now.
I once set up a Saltstack fleet and that worked on a similar principle.
Was great for managing many many machines.