Project Fi relies on existing carrier networks, like Sprint and T-Mobile, combined with WiFi for extended coverage across the US. Google only charges for the data you use with Project Fi, keeping your monthly bill down as low as possible. With the introduction of the Nexus 6P and 5X, that makes the total number of devices that work on Project Fi three. Which while still not the best, is a lot better than it was yesterday.
To learn more about Project Fi, visit fi.google.com.