The Bitwarden mobile application is written in C# with Xamarin Android, Xamarin iOS, UWP, and Xamarin Forms. By default the app is targeting the production API. If you are running the Core API locally, you'll need to switch the app to target your local instance. Open src/App/Utilities/ApiHttpClient.cs and src/App/Utilities/IdentityHttpClient.cs and set the BaseAddress to your local API endpoints (ex. new Uri("http://localhost:5000")). Alternatively, you can also adjust the environment endpoints from the environment settings page on the home screen of the app (log out).