Metro is the JS bundler that serves your app during development and enables Fast Refresh, so most edits appear almost instantly without losing component state.
console.log("balance:", balance);
console.warn("using fallback RPC");
console.error("transaction failed", err);React DevTools (usable via Flipper or standalone) lets you inspect the component tree and props/state live; for native-level crashes, Xcode/Android Studio logs are the next stop.