Google Fi used T-Mobile, Sprint, and US Cellular behind the scenes. At least, they did at the time I signed up, which was why I signed up. Sprint merged into T-Mobile a few years ago; Fi dropped US Cellular last spring.

Since then, I've stayed on from inertia. I'll probably switch to T-Mobile's $15 / 5 GB plan soon, since it's half the price I'm paying for now-no-better service from Fi and I rarely use more than 1 GB per month.

Yes, Google Fi is an MVNO of T-mobile's network.

Basically exclusively at this point.