Multi-Stage Retrieval and Ranking Funnels

Search, ads, and feed systems are constructed as a funnel: retrieve a broad candidate set, rank it with a heavier model, re-rank the top with the heaviest model, then filter and blend with business rules. Each stage trades recall for precision and cost, so cheap models process many items and expensive models process few. AI, ML, and GenAI engineer interviews probe this because it is how every large-scale ranking system is actually built, and because freshness, diversity, and policy constraints have to fit into specific stages.