Engineer
Major
Actually, the MS-21 is full of western sub-systems as well. From , we see western involvement includes Goodrich, Hamilton Sundstrand, Kidde Technologies' L'Hotelier, Liebherr Aerospace, P&W, and Rockwell Collins. Zodiac Aerospace will provide the furnishings, such as seats and wall panels. Your own reasoning would say that Russia's reliance on foreign companies for sub-systems is a sign of the country's inability in making equivalent subsystems; that Russia couldn't even make seats for aircraft.i will tell you why i guessed that.
C919, has many western sub-system, however if China had technically speaking equivalent sub-systems, they could build a Chinese version with only Chinese sub-systems in the same way MS-21 has one russian version with PD-14 engine and one Westernized version of MS-21 with western engine.
Now you might objetc the ARJ-21 and C-919 are civilian aircraft designe for export, however these jets are not being buy by western firms, however the reality they mostly buy by Chinese airliners.
Of course i am guessing, only time will tell
Clearly, your reasoning doesn't work. The fact is that both China and Russia must use western sub-systems out of certification and political reasons. China and Russia can provide all the sub-systems themselves, but do so and the west will not give out certifications. Without certifications, an airliner is useless. Y-20 is a military transport hence has no such restriction, thus doesn't need to involve foreign companies in a joint venture.