Your question is Design System for Billions of Events. 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).
Describe a system you would design to process billions of events per day.
Explain how you would ingest, validate, store, process, and serve predictions from those events. Cover scale estimation, batch and streaming paths, model training and inference, data quality, fault tolerance, monitoring, and cost. State assumptions clearly and discuss how the design changes as volume, latency, or freshness requirements increase.