I. Chinese nationality
Chinese nationality is conferred by the Nationality Law of the People's Republic of China (PRC). It is acquired primarily through ancestry, not place of birth. Under articles 4 and 5 of the Nationality Law of the People's Republic of China, persons of Chinese descent, regardless of whether they were born in China (including Hong Kong) are usually considered to be Chinese citizens. However, there is an exception to this rule: article 5 of the Nationality Law states that a person whose parents are both Chinese nationals and have both settled abroad, or one of whose parents is a Chinese national and has settled abroad, and who has acquired foreign nationality at birth shall not have Chinese nationality. In other words, if one of your parents has settled abroad and you were born in a foreign country and acquired foreign nationality at birth, then you do not have Chinese nationality even if you are a person of Chinese descent.
A child can acquire Chinese nationality at birth even if only one of its parents is Chinese. However, persons who are not of Chinese ancestry are not usually considered to be Chinese citizens albeit they were born in China.
The Nationality Law of the PRC also applies in the Hong Kong Special Administrative Region (SAR). However, it is amplified in Hong Kong by "the explanations", which are an interpretation of the Law by the Standing Committee of the National People's Congress (NPCSC). According to the explanations, some provisions of the Nationality Law may not apply, or may apply differently, to Hong Kong.
Some of the major explanations made by the NPCSC regarding the implementation of the Nationality Law in Hong Kong can be found on the website of the Immigration Department.