Module, service, controller
Build a staff feature the Nest way: a service with the logic, a controller with the routes, and a module that ties them together — then plug it into the app.
Check it like a client
When the feature compiles, test it the way a client would. Request GET /staff and expect the full list; request GET /staff/1 and expect one person; request GET /staff/99 and expect a 404 with a helpful message. Testing the unhappy path is just as important as the happy one, because that's where real users get stuck.
If a route answers 404 for everything, check the module wiring first. If it answers 500, read the server log: an unconverted id or a missing provider is usually the cause.
Tools like curl, HTTPie or the REST Client extension let you save these requests and re-run them after every change.