https://phys.org/news/2018-11-google-telecom-fi-iphones.html

Fi was limited to newer Pixel handsets made by Google and a few Android-powered smartphones made by other companies because devices need to be able to hop between carriers whose infrastructures are used to provide service on the "virtual" network.

Fi "intelligently" shifts smartphone service between Sprint, T-Mobile, US Cellular, and Wi-Fi hotspots to provide optimal signals, according to Google.