good question,
Your understanding is correct, req and res are objects of classes implementing these interfaces,
container creates these objects and calls the servlet's service() method, passing the req and res objects as arguments,
To be specific to your question, regarding name of the classes implementing these interfaces, it depends on the web container you are using.
The implementations of these interfaces are not part of the
J2EE spec, but depends on users of the spec.
Tomcat is considered a reference implementation - i.e. it is considered to be implements the specification to the full and in the correct manner.
In tomcat, Class HttpServletRequestImpl implements HttpServletRequest interface, same way for response.
others- pls correct me if my understanding is not correct