hahaha nice one. Next time try google.
Soviets were not an industrial power at all. Most of their admittedly great achievements were really made possible at the huge expense of destroying the fabric of the rest of their society and consumer industries. Their planned moon rockets used dozens of engines to achieve the necessary thrust and many projects were done through brute force and trial and error. This was brought up as an example to counter the false idea that Russia and Soviet industry was capable. Relating it to Su-57 and others, it's no surprise they can't build high tech stuff anymore. Most of the available technology is based on soviet era technology (which don't get me wrong is still very advanced and many things China still wants to get its hands on). This isn't a insult to Russians. Some of the greatest minds advancing the west in high tech industries are Slavic people born and educated in Russia. All i'm saying is let's not go insane with Su-57 praise. It really aint much.
J-31 is also extremely rough. Even the second prototype. You can see from some available close ups that the frame is like flanker and Su-57, uneven and the naked eye can already detect poor worksmanship in excessive contours. Easily seen under right light. J-10a was same even J-10b was only okay. You can see improvements to later CAC products though and J-20 is F-22 level of skin detail. Not sure about how well that holds up with all the stresses. It's close to F-35 level. This may be relatively superficial for 70% of the time but for VLO, it's quite important and does reflect on the manufacturer's ability at least on how they build and assemble the parts. Already a clear difference below not to even mention the protruding tubes. Left of course is Su-57 and right is F-35.
Above is late model flanker and shows what I mean. This is 4th gen yes but other 4th gens are still much neater. It's not super important for 4th gen but shows what some manufacturers are not capable of. Some parts are naturally recycled and reused to save time and money. Struggling car manufacturers did/do that too.
Below is J-31. If you look closely, it has the same problem. SAC is not that great.