You bought a smartwatch. Within a week, you stopped checking the watch face. The dashboard, the streaks, the weekly chart — all of that lives on the phone. That phone app is called a companion app , and it's the most underrated piece of any fitness wearable product. The watch captures. The phone interprets, stores, and visualizes. This post is a code-first walkthrough of how to build one in React Native. We'll cover the architecture, HealthKit (iOS), Health Connect (Android), an optional Apple Watch bridge, and the sync model that doesn't fall over in production. TL;DR for the impatient: Read from HealthKit on iOS and Health Connect on Android — you cover 90% of wearables without writing firmware. Use react-native-health and react-native-health-connect . Make your backend POST /samples idempotent on the HealthKit UUID. This single rule prevents most sync bugs. The dashboard UI is the time sink, not the integration. Plan accordingly.…