It wasn't on the fly. He created 9 Yang after reading and modifying 9 Yin.
Obviously this process of creation is alot faster than inventing something from scratch.
Just like taking an iPhone and producing 1000 "Hiphones" vs a new mate30 etc.
Z3F didn't get the whole 9 Yang to replicate or modify, he had to invent alot of his stuff.
Now look at YG, he was able to invent something in no time lol.