YouTube
Chunked parallel transcoding, per-title encoding ladders, multi-tier CDN economics, resumable uploads of 100 GB files, dedup-aware view counting, and storage tiering for the long tail.
YouTube Top K
Count-Min Sketch and Space-Saving for memory-bounded heavy hitters, exponential decay for non-stationary popularity, two-tier top-k merging, and why approximate merges are not exact.