Design an answer engine with citations

Design an answer engine: a user asks a question in plain language and gets a direct, current answer written by a model, with numbered citations to the web pages it used, streaming within a second or two, with follow-up questions in the same conversation. Cover query rewriting, search and page fetching, passage selection, generation, how you make sure each citation actually supports its sentence, freshness, caching, a latency budget per stage, and cost at 30 million queries a day.