Google Fi Wireless uses T-Mobile's network to provide its base coverage and an extended "network of networks" that includes US Cellular to provide further coverage for certain phones.

If your phone is compatible, it will switch between network providers automatically depending on which service will provide the best coverage in your area. This is only available for phones "Designed for Fi," however, which includes Google Pixel phones as well as many Android models. 

While you can use just about any phone with Google Fi, iPhones are tricky. Google Fi is optimized for Google Pixel phones and select other Android models certified as "Designed for Fi," meaning that they'll have certain benefits that non-"Designed for Fi" phones will not.

This is significant in terms of coverage, as "Designed for Fi" phones will have access to the full range of Google Fi's network sources, including T-Mobile cell towers, US Cellular towers, and a range of hotspots, but iPhones and non-"Designed for Fi" phones will be restricted to just T-Mobile coverage. 

"Unlimited" data is mostly just a reference to a fixed price per month for access to a certain amount of high-speed data. With each of the two unlimited data plans offered by Google Fi, you can use high-speed data up to a certain data cap before your data speeds will slow down due to deprioritization.

This doesn't affect how much data you can use, it just affects the speed. With the Simply Unlimited plan, your data speeds will be capped after you use 35GB of high-speed data, and with the Unlimited Plus plan your data speeds will be capped after 50GB.