CNBC:

Google Fi (formerly known as Project Fi) is a mobile virtual network operator (MVNO), which means that instead of just using one of the “big four” carriers, it automatically jumps between several cellular networks depending on which has better service. Fi hops between Sprint, T-Mobile, and US Cellular, and also favors Wi-Fi whenever possible, including for calls and texts.

And:

It costs $20 for unlimited calling and texting, and $10 per gigabyte of data. Users get money back for whatever data they don’t use, and data usage over 6 GB is free (though Google will throttle speeds after users hit 15 GB). It also has no roaming fees in more than 170 locations.

This something Apple could do? Is there some contractual agreement that prevents this? Interesting development.