Create green Residential zones to designate where Sims can build houses and move in. As soon as part of your city is labeled as a Residential zone, Sims automatically move in, but it’s up to you to keep them there.
Laying down large blocks of solid green can mean a large population, but if you don’t have enough jobs, power, and utilities to keep all your residents happy, then a lot of those Residential buildings are going to be abandoned. Be smart about where you place homes, and beware air and ground pollution to avoid creating a sick ward.
You decide where Sims can build houses by creating residential zones. Sims will build houses and move into new residential zones themselves (you don't build the actual homes, you just assign the zones)