how do RAG implementations work with generic prompts vs specific prompts? meaning, there are prompts that could easily be answered by the base model itself and

how do RAG implementations work with generic prompts vs specific prompts? meaning, there are prompts that could easily be answered by the base model itself and doesn't require RAG. but some prompts might involve questions about something proprietary where RAG is actually useful. so is the default to just run the RAG search index on every prompt and if it returns nothing then you get the plain answer from the base model otherwise you get the augmented answer?