China immigrated to South Africa and now has a green card in the United States. If it is illegal, which country's law will be used?

Any country that violates the law will be dealt with. If you are not in the criminal country, you can apply for extradition. If you succeed in extradition, you will be tried according to the laws of that country. Having an American green card only means that the holder can have the right of abode in the United States and can freely enter and leave the United States, but it does not mean that the holder has American citizenship. The US government will not protect foreign citizens who hold green cards. If China people immigrate to South Africa and hold South African passports, then the China government will not protect China people who hold South African passports.

If you commit a crime in the United States, you must be tried according to American law. After serving your sentence in America, you will be deported. If you are a South African citizen, you will be sent back to South Africa. If you are a citizen of China, you will be repatriated to China.