During my last visit using GoogleFI, the mobile phone shows a US IP when running whatismyip. So even though the phone is connected to the local telecom 5G network, traffic is segmented and routed back to the US carrier (T-Mobile in AWS US east IP ranges if I remember correctly) before hitting any website. So phones with GoogleFI sim works like a US client with a bit higher latency. This was really nice, but attempting to tether another device to the phone was a highly frustrating exercise as it did not work reliably whatsoever.