As you can infer, the three zone types are heavily related, which is what makes the RCI Meter so important. Industrial zones need workers and a place to ship their goods. Commercial zones need nearby residents to buy their goods of the matching wealth level. Residential zones need jobs and places to spend their money (i.e., nearby commercial zones). Happiness is an indicator of the Sims' basic needs: power, water, sewage solutions, and one more factor major factor based on what kind of zone it is.

What's the Difference Between Happiness and Wealth? You need the best roads possible, high density streets or high density avenues. This density includes all 15+ story buildings and completely specialized factories with a wide diversity of different ones that are much bigger than previous densities. High density is the highest density possible to achieve. You need medium density avenues or medium density streets to be built. This density includes 4-15 story houses, 4-12 story offices/stores, and more advanced factories with some specialization (high-tech, manufacturing, drilling/ mining, etc). They require dirt roads or low density streets to be built. It includes 1-3 story houses, 1-3 story offices/stores, and small, wider apart factories. Low density is the first one when you start a city.

Note that wealth and density are not directly related: you can have low-wealth high-density buildings, and high-wealth low-density buildings, all depending on other factors in your city. 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. Ergo, to have the highest populations and earn some of those achievements, you need to make your people as happy as possible. Zones degrade or become abandoned when the residents or workers are sadder. In regards to density, Sims' happiness isn't simply a factor, or part of the calculation: zones upgrade when the residents (for residential zones) or workers (for commercial and industrial zones) are happier and their needs are satisfied.

Simcity 5 layouts