If the machine is already patented 3 years ago in the USA and if Chinese person patent the exact same machine in china, what happens? what will be the limitations to the Chinese person? And limitations (if any) to the person in the USA. Can the china person start productions and sell the product for profit? within country? to international clients? I'm asking this for educational purposes.
what happens if chinese person filed a patent of a machine which is already patented in USA by an other person?
A US patent provides protection only in the US. It does, however represent prior art so no one should be able to patent the same thing in any other country. I'm not saying it doesn't happen, but the patent examiner should reject the application as not being novel. A US patent does not prevent you from manufacturing and selling in other countries. You do need to be careful because there are often equivalent patents granted in other countries.