NixOS has two main selling points:

1. I can declaratively manage my system. That'd probably the Thing you know about it.
2. but it also uses the Nix Package manager which allows you to install multiple versions of the same program. On Ubuntu, if I update bash from v4.6 to 5.0, it will replace /bin/bash and if any breaking changes were made, any program that has bash 4.6 as dependency won't work anymore. On NixOS binaries are stored in /nix/store with a hash. So bash 4.6 is in /nix/store/hwnfuvshajdbgjajebskhak-bash-4.6 and 5.0 gets installed into /nix/store/638jsvusbhsuksvj76hwlsbj-bash-5.0 This allows us to have programs that depend on a old version of a software installed simultaneously with programs that depend on a new version of it.