Google Fi is a new cellular service with MVNO support as shared below:

Google Fi drops ‘Project’ and adds MVNO support for most Android devices, iPhones

https://fi.google.com/about/

https://en.wikipedia.org/wiki/Mobile_virtual_network_operator

Back in 2015, Project Fi launched as Google’s take on cellular service and has over the years added new features, as well as support for more phones. As suggested earlier this month, the MVNO is now simply called Google Fi and vastly expanding its Android device compatibility and supporting iPhones.

Google Fi Rebrand — The first change today is a rebrand from “Project Fi” to just “Google Fi.” A more modern logo makes use of the four Google colors — though in different shades — and of course dots. Dropping the beta-sounding “Project” from the name also reflects a more long-term commitment to the service by Google. In terms of how the service works for existing customers, nothing is changing with today’s announcements.