How does overpopulation cause unemployment?

The oversupply of low-skill workers - and competition for low skill jobs - forces down those wages. Employers have no incentive to invest in labor saving technology as long as labor is cheap.

Overpopulation does not directly affect high-skill jobs, since most overpopulated nations have low levels of education. There is not fierce competition for high skill jobs, since the supply of such labor is low, so high skill wages do not decline.

That widening gap in wage and job security between uneducated / unskilled and educated / skilled workers is very visible in the USA as well as worldwide.