As far as I can tell, a lot of that magic isn't in the OS that way, as in, they are using "Wifi calling", which AT&T and Verizon also support on limited handsets (ie, Apple only, even though all modern Android phones support it fine.
A lot of their multiple device calling magic seems to be done entirely in the backend.
I'm not saying there ISN'T magic in the OS specifically for Google-Fi, but they seem to be trying to use as much standard magic as possible before using their own.
Although, it'd be nice if a Google engineer actually confirmed if my suspicions are right.
