Create the .env file on the root.
BASE_URL=https://XXXXXXXX
Where the BASE_URL is the debug url api.
Then create the .env.production file on the root too, with the same data that the last, but the BASE_URL must be the production url api.
To run a Android/Ios simulator with development environment, just run:
IOS
react-native run-ios
ANDROID
react-native run-android