You can create your first mobile app on CodeHS by navigating to the Sandbox and creating a React Native program. There is starter code for a simple app with text, more examples in the Docs tab of the editor, and links to the full React Native and Expo docs.
CodeHS uses the Expo app to allow you to view apps that you create. While developing a React Native project on CodeHS, you should have the Expo app downloaded to your phone. Expo is available as an iOS app and as an Android app. When you want to view your React Native program on your phone, open the Expo app, choose "Scan QR Code," and scan the QR code that shows up in the Run tab of the CodeHS editor, like shown below. Once your project is open in Expo, clicking Run Code from the CodeHS editor will update the app.
If you are ready to share your app with others, you can go to the Share tab and click Publish. For more information on publishing a React Native program, see this article.