Experts have called F-35 a fat turkey. When J-20 and F-35 fight, it will no longer be a long range BVR fight due to stealth from both sides. This is where J-20 advantages will be obvious. Its delta Canard design is significantly more manuverable than F-35. Delta Canard design is also better in terms of supersonic manuverablity. So, it can evade BVR missiles better. Being better at manuverability is not just about winning dogfights. Its also about evading BVR missiles. Modern BVR battles are all about firing your missiles and then using manuverability to evade your opponents missiles. This is where J-20 will have the advantage.
J-20 is also significantly bigger, which means it will have a bigger Radar. It will have bigger ECM capability. That means it can detect F-35 much earlier. J-20 also has a much bigger missile in the PL-15, which means it can carry more fuel and thus have bigger range or bigger no escape zone. J-20 also has DAS and IRST, so it can detect F-35 without using Radar.
I think F-35 has absolutely no chance against the J-20. Its a simply handicapped design due to its original mission which was being a second rate fighter bomber. When US designed the F-35, they didn't even think China could be a peer opponent. So, they designed a compromised design focusing on cost reduction. That has made F-35 a terrible Air superiority fighter. F-35 was never designed to be a top line fighter. The top line fighter for US is F-22. But its now terribly outdated. They might be able to upgrade it somehow. But 150 F-22 will do nothing against 1000 J-20 that will be online by 2030.
This is too oversimplified. We don't know enough about these planes and their capabilities, RCS, IR signatures, RAM coats, sensors, armaments, escorts, AWACS capabilities, strategies, tactics, etc., to come to these conclusions.
These "experts" are just guesstimating. Even if they solid modelled the planes like all these "studies" we see online, it still doesn't give the full picture. The solid models do not take into account tolerances and manufacturing imperfections. They don't take into account how clean and well taken care of the or even what type of RAM coat is applied or the quality of riveting. No one can tell what these planes are capable of without having a serial production variant on hand and a multi billion dollar lab to test them.
When you say the J-20 is bigger, and therefore, better, you are assuming both sides are equally capable, technologically speaking. The truth is the US has decades on China in that regard, so even if they are deindustrializing and China is rising fast, it does not constitute reasonable evidence for assuming the US is not capable of producing, or is not currently working on producing a new air superiority fighter that would rival or surpass the J-20 by a significant margin, and mass produce it well before 2030, provided their economy has not collapsed by then.
The F-22A can likely compete with the J-20 in all regards. The F-35 might be incapable of competing with the J-20, it still lays out the foundation for a next generation air superiority fighter to be produced in a relatively short amount of time. The sensors and tech are almost ready and their issues are being optimized, and so is the incredible F135 engine, and the new Adaptive Cycle engines are already being tested.
It is even possible for the US to pull the YF-23 off the shelf and and optimize it for today's tech. That alone would produce a phenomenal fighter with lower RCS and IR signatures than any fighter on the market, but chances are, they are working on something even better.
Here is the only thing we know for sure. The next decade will produce some very tasty fruits for military watchers.
