What is APIRouter in FastAPI and why use it?

It tests whether you understand how FastAPI scales beyond a single file and how to enforce separation of concerns using built-in modularity primitives. APIRouter is a mini FastAPI instance that holds path operations; you define routes in dedicated modules and then include them in the main app via app.include_router, optionally setting path prefixes, tags, default responses, and dependencies at the router level. This keeps domain logic isolated, makes testing easier, and avoids a bloated main.py.