Most visitors from mainland China are people who also own iPhones. Until the X models the iPhone did not support multiple SIM cards so visitors would buy local

Most visitors from mainland China are people who also own iPhones. Until the X models the iPhone did not support multiple SIM cards so visitors would buy local HK prepaid cards which means they are on HK uncensored phone systems and networks. If you are on mainland number with roaming or even a second local HK number attached to your China SIM card you are still subject to censorship because everything data goes back through mainland servers even though your access points are in HK. All the above I know from personal experience - lived in China 12 years and now resident in HK.