You can check.
iPhone - View countries with supported LTE networks - Apple
There are sometimes exceptions made, but for the most part they're dealing with regulatory issues with any kind of cellular communications device. The warranty terms say that iPhones, iPads (assumed to be cellular versions only), and Apple TV devices may have warranty/AppleCare service restricted to the country of sale. And from hearing about how customers have received service, the EEC seems to be considered as one country for warranty service. I believe this is mostly about distributing the entire phone, since many "repairs" are only done as whole device replacement. Even a logic board may be restricted because it has to meet local regulations. That's not to say you can't legally bring a phone to a different region, but for sale that's another matter. If it's something simple like a bad battery or malfunctioning screen assembly then I'm thinking that's not likely to be an issue obtaining warranty service. Those parts are universal for all regions.
Here are the warranty terms that apply to Hong Kong, although it's in pretty much every single set of Apple English language warranty terms.
Apple One (1) Year Limited Warranty
IMPORTANT RESTRICTION FOR iPHONE, iPAD, APPLE TV AND HOMEPOD SERVICE.
Apple may restrict warranty service for iPhone, iPad, Apple TV and HomePod to the country where Apple or its Authorized Distributors originally sold the device.
