Maybe if you are comparing it to the US cost of living? The average manufacturing job for tech companies in China is around 12k usd a year, while a middle class yearly income starts around 15k a year. So they are in the upper end of the working class.

The basic cost of living is just extremely cheap compared to western countries. Although the wages are only 20-25% when compared to US manufacturing, the cost of basic necessities like housing and food are about 3x cheaper.

It really matters what region you are talking about, tier 1 cities are quite a bit more expensive than tier 2 and 3, but still affordable compared to western nations.