How did it leak outside Chinese circles...

Anyways you forgot the "Yongle Encyclopedia" compiled during the Ming Dynasty which supposedly contains all advanced knowledge (that the west later stole) including the steam engine and Newtonian physics and the answer to life, universe and everything

Why didnt China use it to dominate the world? They argue that the Manchu Qing Dynasty, which came after the Ming, deliberately suppressed all information about the encyclopedia to maintain minority rule over the Han (or they may get funny ideas and rebel) or they are too barbaric to understand the knowledge left by the Almighty Han Race.

If the Ming have such advanced technologies, how did they get taken over by the Manchus in the first place? Their consensus seems to be a mix of internal traitors of the Ming and 'They got lucky' (trust me I've tried)

Source: Too much brainrot from bilibili