Let’s put it this way, the Su-27 production equipment was before the disintegration of the Soviet Union, and the huge territorial air defense required a large number of fighter jets and multiple production bases. In the Soviet Union and most socialist countries, there is no such thing as patent rights! At that time, Ukraine, as a member country where military-industrial enterprises gathered, was able to produce anything! After the disintegration, the drawing production line will of course remain local. So it is quite unsurprising that Ukraine produces Su-27!