Live event chat at scale

Design the live-comment / live-chat system for a streamed event (sports final, product launch) with up to 5 million concurrent viewers in one room. Viewers post short comments; everyone sees a fast-moving feed. At peak there are 100k comments/min inbound but no human can read 5M/min, so the displayed stream must be sampled. Comments need light moderation (spam/abuse) before display, and the experience must feel live (<2 s). How do you ingest, moderate, and fan out at this scale?