If the retriever returns zero relevant documents, how do you prevent the LLM from making up an answer?

Explain how you would combine BM25 (keyword) and Vector (semantic) search. What kind of queries would fail if you only used vector search?

How do you handle a "Delete" request in your vector database if a user wants their data removed (Right to be Forgotten)?

Why might you use a "Cross-Encoder" re-ranker after your initial vector retrieval? What is the trade-off in terms of latency?