Back in 2013 China announced that it was investing $16 billion in an effort to fix some of its problems encountered manufacturing high-performance (as in world-class) jet engines. Although China has been working on this for over three decades, continued problems with materials durability, efficient design and quality control have prevented Chinese engines from being competitive with Western models. That large investment appears to have paid off because by 2019 all new Chinese jet fighters were entering service with one of several variants of the WS10 engine.
The WS10 engine has been in development for 30 years so that it could eventually replace Russian AL31F engines used in the Russian Su-27/30 and Chinese clone (J-10/11/15/16) jet fighters. WS10 development efforts encountered one problem after another. Many Chinese engineers considered the WS10 design a superior engine to the AL31F, even though the WS10 copied a lot of Russian technology. The Chinese point out that as delivered from Russia, the AL31 is good for 900 hours of operation. Chinese engineers figured out how to tweak the design of the engine so that it lasted for 1,500 hours. While those tweaks worked on paper, the Chinese were frequently frustrated getting them to work as predicted because their Chinese suppliers could not produce key components to the needed level of quality and durability. Some of those recently invested billions were apparently spent on getting component suppliers to upgrade production techniques and product quality so that this weak link in the supply chain was no longer stalling full-scale production of the WS10.
After 2000 China believed it would be free from dependence on Russia for military jet engines within the next five years. By 2010 it was clear that the “within five years” expectations were not happening and that was a major reason for the additional $16 billion investment. The fundamental problem was that Russia and the nations that set the world standard in military high-tech had developed high-tech infrastructures. While Chinese designers could come up with superior (on paper) designs, the technical skills required to manufacture components and fabricate the actual device were not sufficiently developed to make the Chinese designs work as intended. You needed skilled and experienced workers and managers to get it all right. Within five years China fixed that problem, mainly because many component manufacturers were close to the goal already with the extra money and attention getting them the rest of the way, and able to produce complex components to spec and do it regularly.
Now there are at least seven variants of the WS10 being used in five different Chinese jet fighters. This includes the new J-20 stealth fighter where the WS10 is a placeholder engine until the more powerful WS15 shows up in five or six years. That is one reason why the J-20 is not being produced in large numbers yet.
China recently received some Russian Su-35 jet fighters and their advanced Al041F1 (or 117S) engines. This purchase was made, in part, to get a close look at the high-end tech the Russians have mastered (with some difficulty) to make this engine work. China has long copied foreign technology, not always successfully. Since 2000 China has gradually put more and more money into developing a jet engine manufacturing capability. The Chinese encountered many of the same problems the Russians did when developing their own engine design and component construction skills and that two decades of escalating efforts paid off.
China does have several advantages over Russia in this effort. First, they knew of the mistakes the Russians had made, and so were able to avoid many of them. Then there was the fact that China had better access to Western manufacturing technology (both legally and illegally). Finally, China was, unlike the Soviets, able to develop their engine manufacturing capabilities in a market economy. This was much more efficient than the command economy that the Soviets were saddled with for seven decades. Despite all this, China continued to encounter problems with consistent quality in manufacturing key components. China has overcome those problems and mastered Russian engine building techniques. Now they are slowly expanding the rate of production while moving to eventually surpass the Western firms who have long been the masters of this technology.