Besides: China does not want outside investment.

Belt and Road initiative says otherwise. Most of those debts have to be paid in US dollars and if they aren't paid off, China has in the contracts to acquire lands and infrastructure of foreign countries. Conquering the 3rd world will serve as practice for conquering Taiwan (or if the situation becomes favorable enough with US tied up in multiple wars, they'll try to take Taiwan sooner).

And china has several dead bodies in the closet: So why spent young age males and females on a war where the only outcome is that there is nothing left?

They have too many men of military age from one child policy, their economy is in the shitter, and the CCP needs a distraction and somewhere to put all the extra men to both acquire new resources and serve as sacrifices so as to not rebel against the CCP later.