Stealing and copying or reverse engineering technology has existed for centuries. Industrial revolution in the US was powered by stolen technology from Europe, just read or watch the documentary series The men who built America. With regards to military technology radar was a british invention while stealth was Russian but today the US has perfected the two after espionage or encouraging defectors. The russians basically had people working on the Manhattan project on the atomic bomb as spies and that is how they went nuclear something Truman had bragged it would take them decades to achieve. The Israelis are master reverse engineers too. So basically everyone steals when it comes to technology and you can't use that to say they are inferior. With regards to chinese weapons being untested that is something even the US can't definitely be confident about either. Testing your weapons against a rag tag militia or a weak country like Iraq is nothing compared to an opponent who has the ability to fight back. A sophisticated jet being flown by a third party who is not well trained is as ineffective as a worthless piece of equipment. That is why when the US shows off kills by it's pilots of Russian jets piloted by Iraqi or say Korean pilots it's nothing to brag about. When the same jets were piloted by the russians as is the case in the Korean war the US lost. Korea and Vietnam proved that the US is not invincible. Remember in both cases the US military ran for the first time in history.
Espionage was always present, countries would steal ideas from each other, yes, but they put their own innovation or spin on it. China is just cloning weapons and products shamelessly, fake Apple stores, counterfeit KFC restaurants and imitation IKEAs are common place in China. J-20 fighter is a poor copy of F-35 fighter. They have even built entire replica European towns. Chinese cultural attitudes towards imitation are rooted in Confucianism. We have not talked about the poor quality and workmanship of their work, they haven't been battle tested. America has been testing its weapons systems for decades against inferior adversaries yes but at least it works. Who knows whether those Chinese junks would work as they advertise them.
BTW are you one of those Chinese 50 centers who get paid to spread CCP propaganda. You've been going a little hard in your defense of the Chinese, which is surprising in light of their treatment of black people in China