There's a difference between a data center and a POP. The former is very large. The latter is usually a rack or more in someone else's facilities. When you try to reach Google or Facebook, you rarely hit their data centers directly. They're out in the boonies, where land and power are cheap. Rather, you talk to their proxies at the edge (POPs, often in large cities). From there, your data is either returned immediately (e.g. cached objects such as popular videos or the company logo...) or forwarded over their fiber to the core clusters in the data centers.
Just curious, is there a good site/book/etc to learn how the modern internet actually works? As a lowly programmer, I have a good understanding of network communications, and some knowledge of things like routing protocols, but I'm completely lost when it comes to understanding how the modern internet actually functions. Thanks!
It used to be the case that they were mostly in POPs, but I think that with Maglev (https://research.google.com/pubs/pub44824.html) they can live in core clusters, too. Other Google sources go into more detail, e.g.
Back to your question, I'm not sure there is one good place to look up these things, but presentations/papers by companies like Google and Facebook are probably still your best bet. Stuff coming straight out of GCP teams will be a little more enthusiastic in tone, but that's easy to tune out. :-)
Another good example is Facebook's Ben Maurer and his Fail at Scale talk, which discusses a lot of details that are necessary for modern internet services, such as queuing, session/application-layer congestion control, canarying, advanced monitoring, etc. https://queue.acm.org/detail.cfm?id=2839461
