U write whole system config in a file or few (including grub,ssh,etc) then rebuild system and u have a system based on that config.
There are projects for arch like blendos (the alpha release)
I like it as a concept, but it gets bothersome to maintain on the long run, sometimes you just want to install something not write configs.
I think Gentoo has a nice middle ground, where you can install packages as a one-off without adding them to the world file, which makes it very meat to maintain both your regular packages and some random things you're trying out before settling in on adding them permanently.
That being said I'm currently looking into writing some ansible for kick-starting machines, so I'm very much moving in that direction. Why not use nix then? Few reasons:
Using Nix means I'm forced to use Nix, whereas with Ansible I can use whichever distro I want, more than one even.
I don't want to have to define EVERYTHING, I want to be able to bootstrap systems quickly, but after the initialization I want to be able to mold each system to what I need without worrying about making it reproducible.
Nix uses a language that's only usable in Nix, in short I would need to study and learn something that's only usable on one specific distro.
Nix has an ephemeral command to "install" packages to try out before installing permanently.
nix-shell -p <package> will install the package, and drop you into an ephemeral shell to test it out. Exit the shell and it's gone.
It's also possible to install permanently straight from the CLI, but that ruins composability. To each his own.
My bigger problem w nix is the lack of FHS and the hoops you have to jump thru to get a non standard app to work.
It is pretty great, but for now they are still mainly aimed at power users. I have used home manager for a bit, but I feel some module are not exactly well maintained, and using it is not exactly "maintenance-free". BTW, they pollute your hone dir like crazy, as if xdg has never existed.
I feel like nix is aimed as ease of deployment, but not the ease of maintenance especially for desktop use. However, I love atomic distros, they are on the part of the spectrum, you cannot replicate your setup exactly by copying a dir, but they are very easy to use, with sane default.