It all began with Nix software build system and package manager; they needee a way to build, compile software in a reproducible way. That is, if it builds on my machine, it should build on yours too given some constraints. Then they build a whole package repository for such sofware or package definitions, Nixpkgs, that can be build or retrieved using Nix package manager. Nixpkgs grew to be a repository for enabling runnig an GNU Linux OS on it: NixOS. It is declarative in the sense you write what it should contain like packages and behaves like system services. For example, see https://git.sr.ht/~misterio/nix-config.
Atomic in the sense that when you want to change system's configuration or state, everything should suceed in that update, otherwise fails; it is everything or nothing. This enables storing previous and current system revisions, so can rollback to previous state.
Nix plus things like flakes, nix shell, enables a build inviroment akin to containers, but much better, correct, and flexible.
GNU has interviews and more clarifying all the way back to MIT AI lab, lisp machines the printers proprietary code triggering free software movement, etc.
Linux just happened because GNU hand't developed a proper kernel yet; Linus wrote that himself on an mail to the Linux Kernel or Minix mailing list IIRC.
Just do it like me and listen to nature sounds like gentle rain.