> How do they store their data?
SQL.
> How do their different applications talk to each other?
Proprietary APIs.
> How do they scale their systems to work for millions of users?
T H E C L O U D
> How do they keep them secure?
They just... don't.
> How do they make sure nothing goes wrong?
They just... don't.
> What are APIs, webhooks and client libraries, when you really get down to it?
Easily outsourced to India.