How a Google side project evolved into a $4B company

How did Niantic happen? How did the company behind Pokémon GO and (soon) Harry Potter: Wizards Unite come to be?
When anyone talks about Niantic, they generally mention that it’s “a Google spinout” and move on. As if that’s something that just happens every day. That dozens of people within a massive company come together, build something… and then just leave and take their work with them to form a new, independent company that goes on to have a valuation of