Need help with getting Sims into the world...

I've been watching YouTubers do Sim videos with loads of characters from the gallery and I thought "I want to do that too!". I know how to download characters from the gallery and the like but I have a few major problems...

Whenever I try and add a Sim to the world form the gallery, it gives them 20k to start off with, leaving me with a few shoddy houses for choice. How are YouTubers able to get so many into their games so quickly?

In these same videos, I have seen many Sims who don't appear to have houses. Either they have just never been shown or they're homeless, which I believe is more likely as there are too many Sims for the limited number of lots, combined with the many vanilla Sims still in their game. So how do I get Sims to appear in the world without placing them in a house?