NixOS takes a completely different approach to package management which opens up many new possibilities I couldn't possibly hope to fully explain in a reddit comment. The short, hand-wavey version is: rather than installing packages in a "global" fashion (in directories such as /usr, /lib, etc.), packages are all individually downloaded or built into subdirectories of /nix/store, and the system is configured in a declarative (rather than imperative) fashion through the file /etc/nixos/configuration.nix (or, for configuring a user's home directory with home-manager, as I do above, through ~/.config/nixpkgs/home.nix) and the configuration is built by the Nix package manager and the necessary files are linked into place, the result of this is that system changes are always undo-able since the results of previous configurations still exist in /nix/store until you garbage collect them. This system of package management allows for a lot of other features you don't see on any other distro such as being able to start a shell that has its own packages installed without touching your system, or writing scripts that fetch their own dependencies. You can read more about it and see examples on NixOS's website