Your question is System Design From Scratch. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Design a system from scratch within the following requirements.
Clarify the scope, size the system, and propose an end-to-end architecture. Focus on retrieval, ranking, re-ranking, online vs batch serving, evaluation, monitoring, and failure modes.