Bryan wants to know how secure Google Fi is. Leo says that there isn't any data beyond it, but there's word that all traffic is routed through Google's VPN. So if he trusts Google, then there's little to be worried about. If he doesn't, he should just remember that he still has to trust the carrier.
Here's an explanation from support.google.com on how it's done. If he would prefer to use one carrier over another through Google Fi, then Leo suggests an app called Signal Spy that will tell him what carrier he's using, and give him the option to switch carriers.