Backend and services
Backend infrastructure handles actions that cannot be done within device - things like user authentication, business integrations such as booking appointments, notifications, messages, etc.
Third Party Service Integrations - many third-party services already exist for specific functions like push notifications, mobile app analytics, user authentication, and more. Depending on what kind of services you choose, there may be available APIs that can be leveraged for your app, which is typically less costly than having to build from scratch.