That’s not why they were fired. It was for non performance.

In this case she resigned. I'm just wondering why Google didn't fire her and that's it?

I don't know the specifics of the case but it is often a lot easier to "manage them out" than to fire them. Often this means putting them on a performance improvement plan that you are pretty sure they won't satisfy but effectively tells them to start looking for another job.

No, it's not. California, like most states in the US, is an "at-will employment" state, which basically means they can fire you for just about anything. They don't even need a reason.

But in practice, at least in big tech, they will always want to have a documented and defendable reason for firing people to avoid lawsuits.