It's not 1-3 people. The entire SRE team globally - including the technicians and the engineers with server access - is easily going to be in the hundreds.
The SRE manager is in charge of keeping it all running. He isn't running around the world swapping out servers. He also isn't sitting back with his feet up thinking "All done - now how are my Pokemon doing?"
It's a dynamic process with quality monitoring, budgeting and reports, post-mortems, continual experiments to see if uptime can be improved, and redesigns as hardware and software change.
It's part of the backend, but is only loosely coupled to the content management and delivery system, the ad machine, moderation, marketing, and so on, all of which are going to have similarly complex structures.
