Nix does not require NixOS, and I would even wager that in corporate dev environments it's most often used in a standalone manner.
You can install Nix on Debian/Ubuntu/RedHat, get a reproducible local dev environment and build reproducible OCI images that can be deployed to K8S/Cloud Run/Fargate/etc [0], without ever touching NixOS.
[0]: https://nix.dev/tutorials/nixos/building-and-running-docker-...