Nix works by putting its packages into some installed folder, and then building a system by symlinking to those packages. You'd retain the nix package storage

Nix works by putting its packages into some installed folder, and then building a system by symlinking to those packages. You'd retain the nix package storage between boots (so as to avoid having to redownload/rebuild), and setting the system up just involves setting up some symlinks. - GrahamC's blogpost discusses this in a bit more detail. "Fresh system each run" isn't too exotic compared to image-based approaches like launching a VM on AWS, or running a Docker image.