It is possible that J-20 possibly uses conventional RAM since it a jet of the 2000's and metamaterials R&D is probably going on for 6th generation fighter.
There is a full fledged discussion in this forum ( back in 2018 ?) about the metamaterials in J20. For the sake of not upending the thread with a discussion that'd disrupt the flow I'm linking past posts
#2265
#2096
#2097
The discussions around the metamaterials always eat up pages (Just like Nuclear Warhead threads). It's circular and it's because there is a dearth of information(obvious).
You want to know what I (an internet stranger) think ?
1. China faces no issues with metamaterials research or its fabrication. J20 uses them. How extensive is their use is unknown BUT I know I wouldn't use metamaterials to half ass a result. I'd use them to achieve an intended result ( performance against specific radar frequencies).
2. I rate China's RAM advances to be competitive against US's. Whether F35 or J20 is stealthier will never be known. But the battle will never end as both countries will seek to improve their R&D every year. We know, specifically for J20, that significant attention is given to VLO ( The last time I noticed that would be the shaping of the Twin seater cockpit canopy divider. It was angled and was likely to be made so to deflect radar energy away from the source).
3. The biggest hurdle for both China and US is not technology but costs. One can't plate your aircraft in Platinum, can you? If it weren't for costs, US would be flying a very advanced Fighter jet. If it weren't for costs, China would surprise US and compete. But economics is hard and both countries are striving to achieve better results in the most economic way. Operating costs, acquisition costs, maintainance costs etc impose strict limitations on any warfighting machine. Economics will come to play against Sixth Gen fighters (whatever that be) too. Sadly, that'd mean a lot of revolutionary technology possible will remain unfeasible.
