Nix isn't an OS, Nix is a build system, sort of like Bazel. The way you use Nix as an OS (NixOS) is to make Nix build an OS for you. Every time you install a package or whatever, that's just an incremental recompilation.

So say that you need to make a Docker container running nginx hosting a static website that you build with Jekyll. At a high level you'd write a Nix file like (very much pseudocode):

with let site = buildWithJekyll src={"./src";}; let mynginx = nginx { wwwroot = site; }; docker.buildLayeredImage { inputs: mynginx, entrypoint: "./nginx" }

Nix will run Jekyll to build the site, hash the build output and "site" gets that path. Then Nix takes Jekyll and writes a configuration file that sets wwwroot to the Jekyll output. Then Nix builds a container that runs nginx when it starts. If you just change the Nginx config it won't rerun Jekyll because it knows Jekyll's build inputs haven't changed.

Nix is a build system that runs other build systems. Comparing Nix to Debian doesn't really make sense, it's like comparing build pipelines to makefiles.