react-native-config is a native module library that exposes environment variables from a .env file to React Native applications on iOS, Android, macOS, and Windows. At build time, platform-specific scripts (Ruby for iOS/macOS, Gradle for Android, Node.js for Windows) parse the .env file and inject the key-value pairs as generated constants — BuildConfig fields on Android, a #define'd NSDictionary on iOS, and C++ string constants on Windows. At runtime, a single JavaScript entry point (index.js) calls a synchronous TurboModule method (getConfig()) to retrieve the config object, which is then exported as the default 'Config' constant. The library supports per-environment configurations via ENVFILE or per-build-configuration settings, and includes thorough diagnostics: if the .env file is missing or empty, the runtime logs a detailed message naming exactly which paths were tried. The codebase is well-structured across all four platforms, with a strong test suite for the iOS codegen scripts (Ruby Minitest) and a clean separation between build-time codegen and runtime reading.