Xcode
Apple’s workshop for building, testing, and signing iPhone and iPad apps, and for sending builds to the App Store.
Like a workshop where you assemble, test, and package a product before shipping it. Xcode is Apple’s app for building iPhone and iPad apps on a Mac.
Even with React Native and Expo, you use Xcode to run the iOS Simulator, manage signing, and make the archive you upload. Signing adds a verified stamp that proves the app is yours.
Open the iOS project, pick a device or Simulator, and build. Set your team and bundle identifier, a unique name like com.example.app. Archive and upload to App Store Connect or TestFlight.