Architecture

┌─────────────────────────────────────────────────┐
│                JS (Hermes on watch)             │
│   React 19  →  react-reconciler (custom host)   │
└──────────────────────┬──────────────────────────┘
                       │ JSI
┌──────────────────────▼──────────────────────────┐
│            ReactNativeWatchOSCxx                │
│  Hermes host, TurboModule registry, UI manager, │
│  XHR/WebSocket, event emitter                   │
└──────────────────────┬──────────────────────────┘
                       │
┌──────────────────────▼──────────────────────────┐
│              ReactNativeWatchOS (Swift)         │
│  RNWRootView  +  ViewRegistry / ModifierRegistry│
│         →  native SwiftUI view tree             │
└─────────────────────────────────────────────────┘

The three layers

1. JS — Your React tree, running in Hermes. Reconciliation is handled by a custom host config targeting react-reconciler 0.32.x.
2. C++/Obj-C++ bridge — Hosts Hermes, owns the TurboModule registry, implements JSI shims (fetch, XMLHttpRequest, WebSocket, timers, console), and routes UI commands to the Swift layer.
3. Swift (SwiftUI) — Reads the shadow tree snapshot, looks up each node in ViewRegistry, applies its props via ModifierRegistry, and emits a real SwiftUI view tree from RNWRootView.

Key files

• apple/Sources/ReactNativeWatchOSCxx/RNWHermesHost.mm — Hermes lifecycle.
• apple/Sources/ReactNativeWatchOSCxx/RNWUIManager.mm — JSI -> shadow tree.
• apple/Sources/ReactNativeWatchOS/Registry/RNWNodeRenderer.swift — shadow tree -> SwiftUI.