system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: what happens on the server when one selects to watch a movie on Netflix and how the architecture allows 500k other people to watch the same movie almost concurrently?
gpt
Content Distribution Networks (CDN for short) are used.  Basically, you request to watch a film on Netflix, then Netflix responds with "Sure, here is your film!" - behind the scenes, a CDN that is located near you will provide the actual data for that film to you.  

500k people can watch the same film at the same time from Netflix, but the data that is sent to those 500k people will be coming from 1 of the many, many CDNs that Netflix own.