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.