Design the simplest possible backend: a client that talks to one application server, which stores and reads its data in a database.
Design the simplest complete backend, the shape almost every web application starts as before it grows. A user's client (a browser or app) sends requests to an application server. The application server holds the logic, and it reads and writes its data in a database that remembers information between requests.
This is the starting point for everything else on this platform. Your only job here is to connect the three pieces correctly: the client talks to the application server, and the application server talks to the database. Later designs add load balancers, caches, and queues on top of this foundation, but the foundation is always these three parts wired together.
Lay out the architecture with those three components connected, then briefly describe in the API section what kind of requests the client sends, and in trade-offs, name one limitation of having just a single server.