For J-20 reproduced the whole system of the American fighter.
China recently shot a very convincing video with a J-20 fighter, which was widely discussed in the world media. Perhaps this is a coincidence, but after a while, the video from the Su-57 was published by the Russian Ministry of Defense. In it, finally, the Russian fighter demonstrated the launch of an air-to-air missile from the internal compartment.
The Chinese claim that their aircraft is more ready than a Russian fighter. Also, in their opinion, the J-20 has less EPR, while in the Su-57 stealth paid too little attention. Each proud Beijing intends to produce at least 40 fighter jets, while with the serial production of the Su-57 by the deadlines nothing is yet clear.
It is believed that the J-20 has good avionics and sensors. In 2007, the Chinese “hacked” the Americans and obtained F-35 blueprints, then the attempts continued. As a result, The National Interest notes that the J-20 sensor system is very similar to the Lockheed Martin Electro-Optical Targeting System (EOTS). Eurofighter and Su-57 have different systems. The J-20 and F-35 have it under the nose of the fuselage, and this suggests that the aircraft are suitable for ground attacks.
But Beijing has a serious problem – the engines. Now the J-20s fly on the WS-10B, which NI considers less reliable than the Russian AL-31F. WS-15 is not ready yet, and in terms of engines, the Chinese are more difficult, since they always bought Soviet ones. And copying the engines was harder than avionics.