Excerpted From: https://www.investopedia.com/articles/investing/121814/hong-kong-vs-china-understand-differences.asp
The relationship between Hong Kong and China is far more complex than most people realize. It involves politics, economics, trade, laws, and, above all, the people. "Hongkongers," as they're known, who lived for years under the influence and ways of former ruler Great Britain are wary about China’s intentions and indignant about the mainland’s meddling in its political affairs.
Mainland China and Hong Kong complement each other economically. However, their political differences remain entrenched. The century-long separation between the People's Republic of China and Hong Kong created gaps which cannot be easily bridged even if the two are officially one country. Before Hong Kong and mainland China can truly unify, they must overcome significant differences.