How did Yahoo get hacked?

Yahoo has had two breaches, one in early 2014 and now one in late 2016.
The FBI has now announced that this is the largest breach in cybercrime history, with over ONE BILLION accounts being hacked.
How? Apparently, it all started with an unsuspecting “spear phishing” email to a Yahoo employee in early 2014 (read more about spear phishing here). That makes this the biggest known hack of user data ever, and no other hack comes near to it.