Who that someone is who works on the generic solution, can vary. Sometimes it's one or more of the teams already mentioned. Sometimes, like in this case, it's someone with expertise in related areas that takes the initiative. And a project of this scope invariably gets reviewed on a regular basis by senior engineers, all the way to Urs (who leads all of technical infrastructure). Shared technologies require not just headcount to design and write the systems, but also to operate them (by SREs when they're large enough), so you need to get upper management involved as well.
I'm not close to Google, but from those I know on the product side it can be 'a Gaggle' with nobody really in charge ... but I guess if you have enough self-motivated conscientious actors, and mature people, without ugly turf wars, who can have reasonable discussions, and responsible enough people in charge that can steer things in an appropriate direction ... it works.
But the fact this is an evolution and not a 'new product' is probably prerequisite - so many smart people are hard to coral around new ideas, but if it's done A B C times, then a 'Z' solution speaks to an Engineers sense of efficiency and it should be natural for such an org to want to do it.
I won't name names, but I worked at a large tech company that could not get 'Single Sign On' to work. It was really frustrating to think so many reasonably smart people couldn't figure that out.
We don't need genius I think just a wealth of experience and a lot of common sense.