I read google is having a hardware event on Oct 5th where it might be announced. But if the past is anything to go by, it'll be some weeks after that before it is actually sold.
Fi used to use sprint/tmobile/us cellular and seamlessly switched between the best network based on your location.
But since tmobile acquired sprint, it is only a matter of time before you'll mostly be on tmobile. They aren't officially shutting down (parts of) sprint network until next year
it is more expensive than most MVNOs but if you travel to other countries it is one of the best for roaming, depending on the plan chosen https://fi.google.com/about/international-rates/
at least for tmobile network, google fi has postpaid priority vs most other tmobile MVNOs