Every previous chapter optimised for latency, throughput or correctness. This one optimises for money, and that changes which answers are right.

Do the estimate before anything else. Five million daily users watching five videos of 300 MB each, served from a CDN at roughly $0.02/GB:

5,000,000 users x 5 videos x 0.3 GB x $0.02 = $150,000 per day

Fifty-five million dollars a year, in bandwidth alone. No database, no compute, no salaries — just moving bytes to viewers. That single number outweighs every other cost in the system, and it means a design that is elegant but bandwidth-hungry is simply a worse design.