Once you have your React Native project initialized, you can run
npx 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.
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 11". If you wish to run your app on an iPhone SE (1st generation), run
npx react-native run-ios --simulator="iPhone SE (1st generation)".
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.