Key point
Contract first
An API is a contract between two parts of your system. Write the contract before you code the route.
For a hackathon project, a good API contract says who calls it, what data goes in, what comes back, and what can fail.
Where APIs sit
In your system sketch, draw arrows between client, server, database, payment service, auth service, and any worker. Each arrow that crosses a boundary usually needs an API or a message format.
Do not list every internal function. Focus on the calls another part depends on. Your React app calling your Express server is an API. Your server calling Razorpay or a college ERP endpoint is also an API.