They would use various sources and methods; there would be many different target requirements in such a project.
There is a lot of Chinese HUMINT activity in Russia doing industrial espionage. There are a couple of cases each year that make the news. The same with Ukraine. You can still buy a lot of technology for from defense industry insiders, the mob, or corrupt officials (sometimes a person is all 3).
Also consider the timetable; you could buy anything in Russia during the 1990's* - they could have gotten documentation then, before the Chinese really got into cyber-espionage. Likewise, they could have hired the actual Russian scientists - who's families were starving at the time - to go work in China.
I doubt the Russians even know the full story.
fyi:
http://en.rian.ru/russia/20110708/165082151.html
* Seriously, the Japanese cult Aum Shinrikyo bought for cash - from a Kremlin official - the entire production process for Sarin gas; knowledge they then used to gas the Tokyo subway system in 1995.
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