In Bong Joon-ho's Parasite (2019), the Kim family (the poor family) were employed by the Park family (the rich family) in the Parks' mansion. At the end of the movie, it was revealed that the mother Chung-sook and the son Ki-woo were still living in the semi-basement apartment.
So my question is: did the Kims get paid by the Parks? If so, why were the poor family still poor?