Stolen or handed to them by American companies that hire foreigners?
Handed.
One of the conditions of opening a factory in China is the handing over your patents to the Chinese government.
I am very familiar with a large American MNC operating in China. In my exposure and work with them, we had at least 3 high-level overseas Chinese employees who were secretly consulting with, or selling advice and experience (secrets?) to local Chinese enterprises, who were competing with my employer.
For their part, this American company stupidly pursued management localization in all its Asian operations, for both short-term cost savings, and giving the company a Chinese face. Thus they faced this problem, again and again, over and over.