You'd think a company like Google that just cancels entire products randomly (https://killedbygoogle.com/) would be smart about layoffs but they probably do it like every other large company. Plan from the top with a target percentage, track demographics of those being let go to ensure there aren't age or other discrimination happening and hit the HR, Marketing, Legal, Finance, IT departments heavier - they are cost centers so its an immediate savings without impacting products. First and even second line managers are probably not involved until it happens and then they get told they have a message to deliver.