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.