I am curius how refund works? Is there a "master key" that allows Facebook customer service to reverse a transaction or create a transaction from an arbitrary a

I am curius how refund works? Is there a "master key" that allows Facebook customer service to reverse a transaction or create a transaction from an arbitrary account? And how does this play with current financial legislation. Do they have a banking license? And where is the blockchain stored? Otherwise great news; I am looking forward to something that probably has a very good end-user experience compared to what else is out there.