React Three Fiber (R3F) is a React renderer for three.js, enabling declarative construction of 3D scenes using JSX. It provides a `<Canvas>` component that bootstraps a WebGL renderer, a custom React reconciler that bridges React's component lifecycle with three.js objects, and a rich set of hooks (`useFrame`, `useThree`, `useLoader`) for imperative control. The architecture follows a monorepo pattern with three packages: the core fiber library (web and React Native), a test renderer for unit testing 3D components, and an ESLint plugin enforcing performance rules. The core library abstracts WebGL details, handles pointer events with raycasting, manages the render loop, and supports advanced features like portals, Suspense for async assets, and context bridging. It is a foundational tool for building interactive 3D applications in the browser or on mobile via Expo. The codebase demonstrates high craftsmanship with clean abstractions, extensive test coverage, and a modular design that separates concerns across reconciler, event system, state management, and rendering utilities.