Your question is Auto-complete System Design. 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 search auto-complete system (similar to LeetCode 642) where the number of returned suggestion sentences is a variable K, entries older than 6 months automatically expire, and the system can scale to a large search engine.
Asked in the Phone Screen stage. This combines data structures, algorithms, and system design for large-scale auto-complete with time-based decay.
Address the data model, prefix lookup and ranking algorithm, expiration strategy, online serving path, ingestion and update pipeline, capacity assumptions, and monitoring. Discuss how you would handle hot prefixes, stale data, service failures, and training-serving consistency if ranking becomes learned.