Software and organizations are always much more complicated than you would realize as an outsider.
Most of those 10k employees are probably not software engineers. Businesses have many other functions.
From a software standpoint, any system that support 100M clients with rich functionality is going to be shockingly complex.
Some companies, notably Google and Amazon, have maintenance of customer trust as a core principle. This seems to be working. Generally, people trust a lot of their private data to these companies and the companies do not abuse it.
Microsoft believes in this too, but had to learn the hard way back around 2000 that customer trust matters a lot.
Facebook seems to have had a harder time learning this lesson. Perhaps they've learned it now, but it'll take a while before consumers really believe it.
Yes, every group in Amazon has its own recruiters who largely find candidates independently.
However, Amazon does have a good central database of everybody who is contacted, who last talked with them, what happened with the contact, etc. And they're usually pretty good about having only one group active with a candidate at a time.