Commercial zones need nearby residents to buy their goods of the matching wealth level.
Industrial zones need workers and a place to ship their goods.
As you can infer, the three zone types are heavily related, which is what makes the RCI Meter so important.
Wealth meanwhile is a measure of the desirability of the area, which means that the extras are taken care of.
While a given zone's maximum density is tied to the type of road the front door faces, its current density is purely based on how successful you're running the city and how happy they are being there.