Live stream ingest and distribution

Design a live-streaming ingest + distribution system (think a creator goes live and 2M viewers watch) with sub-5 s glass-to-glass latency for the low-latency tier, adaptive bitrate, and the ability to absorb a flash crowd when a stream goes viral. Decide the ingest protocol, the transcode/packaging stage, and how you fan a single source stream out to millions without melting the origin. Where's the hard part, ingest, transcode, or distribution?