From my personal experience, you don’t get interviewed for or by a specific group for "ordinary" Google positions. As I understand things from the outside (perhaps a Googler can confirm?) a candidate goes through the interview process & if they make it through then they go into an internal pool of potential candidates who are then allocated where they’re most needed. I interviewed for an SRE position, so I could have ended up attached to a whole range of Google projects.

I would imagine the intention is to prevent teams picking people who are "just like them" (or any of the other ways in which project groups can go a bit pathological inside a large company). By carving out the interview process as it’s own separate thing in this way you can prevent individual parts of the company going off the rails.