[MadnessASAP]

Yes! Nixpkgs straddles both worlds, like a system package manager it provides a way to install packages and their dependencies. However, like most language package managers it also imposes a locking mechanism so that every input into the nix expression* is locked to a hash, the mechanism is recursive and handles multiple versions of the same package in parallel.

The recent(ish) concept of "nix flakes" means there are 2 related but different mechanisms for achieving this but the end result is the same.

* In the land of NixOS everything is a nix expression including the system packages, configuration files, install image. It's all locked to hashes of upstream sources and is in theory, fully byte-identical reproducible.