A REST API request travels through multiple layers before a response is returned.

Consider a user registration request.

1. The frontend sends an HTTP POST request.
2. Express receives the request.
3. Middleware parses the JSON body.
4. The router identifies the matching endpoint.
5. Authentication middleware verifies the user if required.
6. Validation checks the incoming data.
7. The controller handles the request.
8. The service performs business logic.
9. The model interacts with MongoDB.
10. The controller returns an HTTP response.

This separation makes applications easier to maintain and test.