A watch is not a tiny phone. People raise a wrist for a glance, one action or a short live session. If the wearable version begins as a copy of every mobile screen, it is already too large.
Start with one job: begin a workout, check a streak, capture a phrase or see the next step. Then decide where that job should live — on the watch face, in a quick surface, or inside the full watch app.
Apple Watch
A modern watchOS app is built with Swift and SwiftUI. WidgetKit powers complications and Smart Stack widgets, while Watch Connectivity moves messages, state and files between the watch and its companion iPhone app. A watch app can also talk to your backend directly when the feature needs to work without the phone.
For an Expo or React Native phone app, treat watchOS as an additional native target. Share the account, API models and product rules, but expect to write the watch interface in SwiftUI and bridge only the data the two apps actually need.
Wear OS
Wear OS uses Kotlin and Compose for Wear OS for the app UI. Complications provide watch-face information, while Tiles offer a fast surface for a little more content and action. The Data Layer API can sync with a paired Android phone, but an offline-first watch app should store its own state and use the network when it can.
Share the product, not the pixels
Both watch apps can share the same backend contracts, analytics events, copy and feature rules. They should not be forced into the same layout. The Digital Crown, rotary input, platform navigation, complications, Tiles and background limits all deserve native behavior.
Ship one narrow flow on one platform first. Measure whether people use it from their wrist. Then bring the proven interaction to the other watch, translated for that platform instead of merely ported.