(1) people who make their bosses look bad are first to go.
(2) next are people who are perceived to cost their bosses time, regardless of whether it's their fault.
(3) after that, it's usually the infanticide cases: i.e., the people who did nothing wrong but haven't been there long enough to establish themselves.
The infanticide is especially ugly, because (a) it means the company is firing people basically at random, and (b) it rapes the shit out of the resumes of the people affected, because they now have <1 year jobs to explain. The reason it happens is that, empirically, most of the people cut in mass stack-rank purges are new members of underperforming teams... who, by inspection, have had the least to do with the team's underperformance.
Underperformance does get people fired, but rarely. It's at least as likely to get someone promoted, because underperformers usually have a career's worth of experience of being shitty, and therefore have developed such political skills they can easily fail up every time.
Not at Google, you can't. Firing someone who's performing abysmally still takes up to 6 months, between all the process and PIPs and paperwork and shit.