Whenever such big takeovers happen, the monetary settlement can happen in a range of ways. Facebook shelled $19 billion to acquire WhatsApp.
In the report available in the public, it was decided in the agreement between the two parties that Facebook would offer WhatsApp’s owners $4 billion in cash, $12 billion in Facebook Class A common stock, and $3 billion in restricted stock units.
Now, indeed $4 billion is a huge amount. But even such transfers happen like the regular ones we do from one bank to another. Facebook has a lot of liquid assets, as is shown in its balance sheet. They just wired this money to the given bank account of WhatsApp company/owners. Of course, given such a large sum, to avoid any confusion and to ensure things are great, the bank and legal representatives of both the parties were informed and reported to.
As for how the shares were settled, it was exactly how it’s regularly done. But then again, given it’s such a big amount, both parties who have included the best legal team on their sides to ensure a smoother ride. This is exactly how it went when Facebook purchased Instagram. And the similar process was followed in other major takeovers like when Microsoft acquired LinkedIn.