Foreign SIM cards on globs roaming are not passed through the GFW as traffic is carried back to the ‘origin country’ before being sent to the internet writ large. So IP lookups still think you’re in your original country. I used global roaming when working in China to circumvent the GFW (albeit at high expense for data).

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.

From what I read, the way mobile data roaming is designed is basically like a VPN that connects you to your own home provider. Thus if you're roaming in China you don't get the renowned GFW hospitality. That's why some local people use foreign / HongKong sim cards for GFW-less mobile data.

I used Google Fi in early 2020 in China.

iirc it really just depended on where you were... Sometimes Fi let me break through the GFW, but it definitely did not in Beijing.