A very well known fact is that when the Europeans left the cities of 🇭🇰 Hong Kong and 🇲🇴 Macau to 🇨🇳 China, China had to promise to continue to run them under the capitalist system brought over by the Europeans, and let them mostly govern themselves. This became known as "One country, two systems". The two cities are mostly treated as independent nations. They're still part of China in form of "administrative regions".
A not well known fact is that this independence has an expiration date. The contract is limited to 50 years. For Hong Kong, expiration is set in 2047, for Macau it's 2049. Nobody knows what exactly happens by then.