How do I build a react-native app so that it runs as an ipad app on an ipad?
I think you have not kept your Devices in Xcode as Universal. Go to Xcode click on the project name. In the target section go to Deployment Info. Set Devices to Universal.
you can use this command
react-native run-ios --simulator="iPad Pro (9.7-inch)"
Visit https://facebook.github.io/react-native/docs/running-on-simulator-ios.html