Multi-Stage Retrieval and Ranking Funnels

Search, ads, and feed systems are built 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 handle many items and expensive models handle few. Applied-AI interviews probe this because it is how every large-scale ranking system is actually structured, and because freshness, diversity, and policy constraints have to slot into specific stages.