That's because FAANG has a hiring department, that's responsible for hiring. They also each have ~2,000 engineering teams, some of which may or may not have headcount. And once someone is accepted, teams with open headcount look at that person, and make an offer.

The shortens the time that it takes to fill headcount, because there is a constant pipeline of ready candidates.

This also means that you won't get yanked around at the eleventh hour of the interview process with a "Sorry, we know you did great on all your interviews, but the headcount for the one team you were interviewing for filled/canceled earlier today, don't let the door hit you on the way out."

You get 1-5 different team offers if you pass the interview. You can speak to the managers of those teams, prior to saying yes. Those managers, if they are any good, should have a pretty good idea of what you'll be doing. If you don't like any of those teams (or their managers), you can tell your recruiter that (I said no to all 5 of my initial offers, and got 2 counter-offers), and they'll come back to you with different interested teams.

Your recruiter gets paid if you get hired. If you don't care what team you get slotted in, they'll slot you into whatever's available first. If you've passed your interviews, but do care what team you're gong to work with, they are financially incentivised to work with you.

Or you could just switch teams after 12 months, if you don't like the one you were hired into.