Thanks to Expo, you can run your app on your mobile device without installing xCode or Android studio. In fact, Expo will even build your apps in the cloud for you as well, for free.
However, working on a mobile device can be slow and cumbersome so you may want to install xCode and/or Android studio for the purpose of testing your app in an iOS or Android simulator and avoid twisting your neck by constantly looking up and down at your phone.
Gravity Server
As part of your purchase, your app includes Gravity Server – which you'll need to have running in order to manage the server-side processes that your mobile app requires, such as the API, authentication, user management etc.
Database
You will need an empty SQL or Mongo database before running the setup process, so go ahead and fire up a new database using your favourite database service.
If you'd like to use the iOS simulator on your Mac for testing your app, or you'd like to get your hands dirty with some native code – got ahead and download xCode (it's free).
Apple Developer Program
You can run your app on your simulator or device using Expo without a developer account. However if you want to publish your application to the app store or invite Beta testers to install it on their device using TestFlight, you'll need to join the Apple Developer Program, which costs $99 per year.
Developing for Android
To run your app on the Android simulator, you'll need to download a free copy of Android Studio. Publishing your app on the Google Play store will require that you register as a developer with a one-time fee of $25.