If China was so strong why did a large country of hundreds of millions of people get conquered by a few hundred thousand Manchus?
They were a very weak civilization.
Because it is easy to lose to a few hundred thousand well-led unified warriors if your government happens to be degenerate. The Manchu/Jurchen conquered the cowering Ming.
Exactly the same thing happened to the Roman Empire in the fifth century - its population and resources were much larger than that of the barbarians that “overran” it. It does not mean that ancient classical civilization was “weak”.
To pose you a counter-question, if Chinese civilization is so very weak, why are all it’s historical enemies dust?
The Huns are dead and forgotten. The Mongols are weak and irrelevant. The Manchu have ceased to exist. The Tibetans and Uyghurs are marginalized and subjugated.
Anyway to answer your stupid question, and it is a stupid question since you obviously know little about Chinese history during the late Ming period, victory is determined by combatants and not observers. For better or worse, those tens of millions weren’t really relevant as they didn’t participate. This wasn’t a total war where all the resources of the state could be mobilized, but rather China was in a state of semi-anarchy. The Ming dynasty was already being torn apart by civil war prior to the Manchu invasion and the last legitimate Ming emperor had killed himself when rebel forces had taken Beijing. This resulted in a power vacuum where no one was sure where authority in China ultimately lay. Nurhaci was close by and acted quickly and opportunistically and gained the allegiance of several critical disenchanted Ming commanders (and their armies!) and quickly retook Beijing and proclaimed himself the new emperor. From there on, inertia took care of the rest as the remaining military forces threw in their lot with the Manchus before a cohesive Ming loyalist movement could emerge.