Difficult to say for sure when it comes to avionics and electronics between Russia and China. I'd wager that when it comes to radar, Chinese radar technology is ahead in aviation and naval radars. Of course China's fundamentals were build off Russian/Soviet technologies but nowadays encompass a greater variety and depth. China has fielded various airborne and naval AESA and dual bands for close to a decade if not longer now. Over the horizon? Maybe Russia has a few more advantages there.
Chinese electronics manufacturing in general is certainly far ahead of Russia's, but this doesn't necessarily read directly across to better avionics in particular. The requirements of consumer electronics and aviation (civilian and especially military) are sufficiently different that system architecture, non-COTS bespoke circuitry and software play a prominent role. Pure transistor density is less relevant - in fact due to hardening requirements, the chips in these applications are usually several generations behind the latest consumer products. The USSR was constantly far behind the US in consumer electronics, yet managed to keep pace quite well in military avionics, so it's hard to see how China pulling ahead of Russia would be much different. In fact, the recent sanctions not withstanding, Russia has better access to COTS electronic components & know how than the USSR did, not least because of China's rise (but also via South Korea).
Where the superior Chinese electronics base will provide advantages is weight, power consumption and cooling, but again those are issues the Russians always had to contend with. The APG-63 weighed roughly half what the N001 did at comparable performance (depending largely on which variant and time period), but was the F-15 a better fighter for it? There are ways to compensate, both directly (weight) and performance (e.g. IRST + data in the case of the Su-27).
Not sure why you consider the MC-21's wings to be superior. The composites? C919 is similar. On manufacturing of frames, I think China is well ahead and approaching the US.
Not true. The C919 has a conventional wing of largely metal construction with composite secondary structure (winglets, control surfaces, high-lift devices, fixed trainling edge), much like the A320.
On the other hand the MS-21 has a complete composite wing box (including fuselage carry through) with upper & lower wing skins as well as front & rear spars manufactured out of carbon composite, like the 787 and A350. Moreover, the production process is actually more advanced than these Western examples in that for the first time in a primary structure application a pressurized autoclave is no longer required for curing. Instead, the procedure takes place at atmospheric pressure in a far cheaper oven, enabling the process to be scaled up at lower cost for the much larger C929.
Also, the MS-21 uses modern Aluminium-Lithium alloys for the fuselage (like the A220), not the traditional materials selected for the C919, and consequently offers a higher cabin altitude. The C919 does have a composite tail cone, but at best that partially compensates for the lack of a Al-Li fuselage - aircraft consist of a lot more wing & fuselage tube than tail cone by weight, so the benefits are limited.
Look at the J-20 compared to the Su-57's body. Even the J-10C has smoother surfaces, fewer gaps, and seemingly more consistency. Of course this is a relatively superficial way to judge but Russia always approached making planes like building tanks as opposed to the American way, building fine mechanical watches. J-10C, J-20, WZ-8, GJ-11 (actually any modern drone) seems to have better assembly quality.
As for surface finish, the Su-35 is pretty damn good, as is the new crop of UAVs (Orion, Altius and Okhotnik) which will become the counterparts to the Chinese Wing Loong series and GJ-11. Nevermind the Su-57s, which have a mirror like finish in primer. With those UAVs (both Russian or Chinese) in particular it's not that much of an achievement anyway, they're practically entirely composite, so the parts essentially come out of a mold. WZ-8 isn't particularly impressive in terms of finish actually, the matt black paint just hides imperfections extremely well (though of course the TPS might have something to do with it, too). Certainly, close-ups of J-31 v1.0 disclosed rather poor fit and it took years before the existence of internal weapons bays on the Su-47 became apparent - so be careful with surface details on all-black aircraft (that includes the F-117, BTW)!
It's the other way round.
Su-57 horizontal stabilizer has only its elevator part of it movable. The J-20 counterpart of it, the canard is all movable.
Su-57 vertical stabilizer is also partly movable. i.e. the rudder part of it.
Compare this to the J-20 vertical stabilizers which are all movable, and are highly slanted so that it can have some effect of an elevator function as well.
Overall, I would think that the yaw and pitch control of the J-20 is more complicated than the Su-57, especially because of its unconventional all moving highly slanted horizontal stabilizers.
???
You are either confusing the Su-35 for the Su-57 or you need to look again at a Su-57 photo - closely. It has both all-moving horizontal (as indeed have all supersonic aircraft since the 1950s, excepting pure deltas without any h-stab like the Mirage series!) and vertical tails. Moreover its vertical tail area is more dramatically reduced than the J-20's (no ventral strakes either), likely making FCS design significantly more challenging.
Add to that TVC and LEVCONs...
