Jovian-NixOS is a NixOS overlay and module collection that enables running NixOS on the Steam Deck and similar handheld gaming devices. It provides hardware-specific kernel configurations, firmware update integration, Steam Deck UI (Gaming Mode) support, audio/display driver patches, and a suite of system services (power management, input handling, performance control) modeled after SteamOS. The repository is structured as a Nix flake with a clear separation of concerns: NixOS modules under `modules/` define configuration options for Steam Deck hardware, Steam client integration, and SteamOS-like system settings; a Nix overlay under `pkgs/` provides patched or vendored packages (gamescope, mesa, linux kernel, pipewire, etc.) that are not yet in upstream Nixpkgs or require Steam Deck-specific modifications. The codebase demonstrates strong NixOS engineering practices: modular design, extensive use of lib.mkIf/mkMerge for conditional configuration, hardcoded path patching via replaceVars, and resholve for shell script safety. The documentation system is a custom Ruby pipeline that generates a static site from markdown with options and package listings. While the repository is tightly coupled to NixOS and the Steam Deck domain, it is a well-maintained, production-quality example of how to extend NixOS for specialized hardware.