There are probably many such components; I'd imagine SRE alone would be 200+ people

How many of the remaining staff have the knowledge required to keep all of those components running smoothly?

You might have 200 different apps (hell, we have close to that, only 3 people in ops) but competent team will make sure they deploy in same way and are monitored in same way.

And once you go from "a server" to multiple servers, whether the end number ends up being 20 or 200 isn't that important till you start hitting say switching capacity, and if you're in cloud that's usually not your concern anyway.

Our biggest site (about dozen million users, a bunch of services and caching underneath, few gbits of traffic) took zero actual maintenance for 2022, "it just works", any job was implementing new stuff. It took some time to get to that state but once you do aside from hardware failures it "runs itself"

Nobody is adding changes that blows out the DB? or add some inefficient code that burns CPU much faster?

not to be flippant but you find that out like 3 environments below production.

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.