On 1 July 1997, Hong Kong became the first Special Administrative Region of the People's Republic of China, under the principle of "one country, two systems" (the other, Macau, attained that status in December 1999). It has a different political system from mainland China but it is part of the mainland.
And Hong Kong is covered by a separate bilateral than the one for the rest of China.
Indeed it is but that is separate from the discussion over whether it was part of China.
The two Special Administrative Regions (SAR), Hong Kong and Macau, are run as separate entities from the mainland. As mentioned above it is part of the "one country, two systems" agreements between China
PR and the UK/Portugal.
Travel between China mainland and the two SARs and between the two SARs is not classed as domestic, passports are needed.
Perhaps a better analogy is the UK and its self-governing overseas territories (e.g. Bermuda, Cayman Islands, etc.).