Most React Native devs use these five pieces every single day without knowing how they connect. Let's fix that. 🏗️ Build time — Metro Metro is your only build-time actor. It takes your JSX/TS files, transforms them to plain JavaScript, resolves imports, tree-shakes, and bundles everything. In dev mode → raw JS bundle. In production → Hermes pre-compiles it to bytecode before it ships inside the app. Think of Metro as Vite or Webpack, but purpose-built for React Native. ⚡ Runtime: JS layer — Hermes Hermes receives the bundle and executes it on device. The key difference from V8: Hermes is AOT (Ahead-of-Time) , not JIT. By the time the app launches, the bytecode is already compiled. No parsing, no JIT warmup — execution starts immediately. That's where the startup speed win comes from. 🔗 Runtime: The bridge killer — JSI When JS needs to talk to native code, it used to go through the old bridge — JSON-serialized messages passed asynchronously. Slow, unpredictable, and batched.…