The server in this project is quite contrived (as is the project itself). This is intentional. The goal of this project is to show you an example of how you might configure a typical client-side application for testing. The server aspect of a client-side application varies widely from app to app, so keeping it as simple as possible is an effort to avoid distracting you from implementation details of this specific app and help you focus on what really matters for configuring these tools.
Suffice it to say: When E2E testing an app with Cypress, you'll likely need to start the backend server in addition to the frontend app. This project will demonstrate how you might accomplish that, but the specific scripts you run to do that could vary.