I didn't know the full extent before I started, but they threw more money at me than my wife and I have ever made combined, so I probably would have still taken it if I had known. Plus, I'm in NYC now.
I see no conclusive evidence in the parent comment that proves it's Google. I would've actually guessed Apple.
My understanding is that Google actually has a reasonable process for this, and that people have taken advantage of it to start or contribute to projects in a personal capacity.
They do approve side projects in rarer cases where it has nothing to do with Google's business - for example, my friend recently wrote an app for sex positions. But most things that I want to work on have a messaging / ML / AR component to them somewhere so I haven't felt comfortable building anything while I've been employed there. Hence the main reason I'm leaving Google at the end of the month, to build those things :)
1: Technically, the policy is "ask your manager" but everyone knows what the answer is.