When I run this command:
react-native run-ios
My app runs by default in the iPhone6 simulator device:
Found Xcode project R
You can also use npm for this by adding an entry to the scripts element of your package.json file. E.g.
scripts
package.json
"launch-ios": "react-native run-ios --simulator \"iPad Air 2\""
Then to use this: npm run launch-ios
npm run launch-ios