Starting the simulator
Once you have your React Native project initialized, you can run
react-native run-ios inside the newly created project directory. If everything is set up correctly, you should see your new app running in the iOS Simulator shortly.
Specifying a device
You can specify the device the simulator should run with the
--simulator flag, followed by the device name as a string. The default is
"iPhone 6". If you wish to run your app on an iPhone 4s, just run
react-native run-ios --simulator="iPhone 4s".
The device names correspond to the list of devices available in Xcode. You can check your available devices by running
xcrun simctl list devices from the console.