HK is part of China. How do you send someone from China to China.
The _system_ in Hong Kong is very different.
It isn't uncommon to have such internal differences. The United Kingdom of Great Britain and Northern Ireland is likewise a single sovereign entity, often abbreviated as just "the UK" but for example abortion is _a crime_ in Northern Ireland, whereas it's offered as a normal part of the NHS on the mainland. Likewise Scotland has separate laws even though there's no discernible border, no more than a small street sign saying "You are now entering Scotland".