Massive livestream comments
Design the live-comments/chat system for a massive livestream. A single stream can have 2M concurrent viewers; at peak (a goal, a product reveal) tens of thousands of comments per second are posted, and every viewer wants a live-feeling comment feed. You obviously can't show all comments to everyone, and you must handle spam, slow-mode, and moderation in real time.
What a strong answer looks like
Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts (data model, bottlenecks, consistency, failure modes) and name the trade-offs you are making.
Clarify5:00 left
Estimate5:00 planned
Design15:00 planned
Deep dive12:00 planned
Failure8:00 planned
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.