It is a little unreliable. Often traveling to the actual neighborhoods can trigger some of the houses being filled. Other times talking to out of world sims at a park or some other community lot can trigger those sims being placed into a house. It generally happens over time and usually no more than 5 houses across all worlds.
When you are in live mode, if your sims clicks on a empty house there is an option to fill that house with sims from your library. That is often a good quick way to add more sims into your worlds and straight into houses. I have a stash of townies that I made for this purpose. I randomized their traits when I made them so it feels more like 'game generated townies' I'm using rather than 'hand picked' sims.