A:Answer (Adding to answer by md2572.). Google Fi mobile network service also works via Wi-Fi*. It can automatically switch/select the available network with best signal/quality. Between the supported 3 cellular networks (T-Mobile (GSM), Sprint (CDMA), US Cellular (?)), & Wi-Fi (* access points on their network service).
The devices supported/allowed on Fi is limited, especially for full Fi service. Some Moto X4 & Moto G6 editions are included.
For more info, check Google Fi, & also trustworthy review/testing/comparison sites.