Today I Learned

ReactNative: How to setup Bugsnag stage in Native files

you need to use react-native-config and setup it for each schema/Android flavor (eg. .env.demo)

...
ENV=demo
...

iOS

inside file AppDelegate.m put

  NSString *envFromConfig = [ReactNativeConfig envFor:@"ENV"];
  BugsnagConfiguration *config = [BugsnagConfiguration loadConfig];
  config.releaseStage = envFromConfig;
  [Bugsnag startWithConfiguration:config];

instead of [Bugsnag start]; inside didFinishLaunchingWithOptions

Android

inside MainApplication.java put additional imports at the top of it

import com.bugsnag.android.Bugsnag;
import com.bugsnag.android.Configuration;

then inside onCreate put

        Configuration config = Configuration.load(this);
        config.setReleaseStage(BuildConfig.ENV);
        Bugsnag.start(this, config);

instead of simple Bugsnag.start(this)