Google might be blocked in mainland China, but is it blocked in Hong Kong? This page [0] says that Google isn't because HK is not behind the great firewall, but there are recent reports that the GFW was extended to HK as well [1], so maybe that page is outdated? But if Google had been blocked at a network level, then it would likely have made much larger headlines. Also, the author talks about Google play still being a present thing, not something unavailable to them.
GFW-compatible fallbacks are worth worrying about in HK, though, because in normal times there is a massive amount of cross-border travel between HK and SZ. Combine that with grey market HK phones being smuggled onto the mainland to avoid tax, and it's a pretty good bet that any phone destined for HK will makes its way across the border at some point.
If you roam on the mainland with a HK SIM you actually hop the GFW automatically, but most people swap SIMs at the border so that isn't reliable.
I think the traffic gets proxied (not sure if it's encrypted) back to servers run by your home carrier. So conversely if you use a China Mobile SIM in Hong Kong or overseas your traffic gets firewalled.
[0]: https://www.comparitech.com/privacy-security-tools/blockedin...
[1]: https://www.theguardian.com/world/2020/jul/08/china-great-fi...