I was looking into downloading a custom world and noticed that some sites say they can come populated or unpopulated. I'm confused on what unpopulated means? Does this mean that there are absolutely no people in the world? If you went out and about to the park or something, then you would not run into others? Or does it just mean there's no sims living in homes? If it does mean there's zero people anywhere, are you supposed to make all the people from scratch and move them into homes? I thought they were just random people generated by the game so I'm not sure how to populate a world. Anyways any help would be very lovely and thank you!

"Unpopulated" means no people at all, the same way "empty" generally means that lots are placed but nothing is built on them. That doesn't mean you have to make people, though, if you don't want to. Other than your initial playable, of course. The game will generate Sims to, for instance, tend bars and run cash registers and to be mail carriers, newspaper deliverers, police officers, etc. Story progression will also immigrate Sims to live in houses, if you don't turn story progression off. (I prefer NRAAS's Story Progression over EA's, though. That way I can dictate the kinds of Sims that will immigrate and when and how many, etc. It's a huge mod, with a learning curve, but IMO it's well worth the effort to learn to use it.) If you have Sims in your bin, you can also move them into houses as townies, if you prefer.

I've played empty and unpopulated worlds along with blank oned mostly and I usually have to with my heavily Modded game and heavily altered NRaas settings move my own families in if I want other residents to be there to show up at community lots when my currently active family goes out.I start playing my worlds once my first family is moved in and start gradually adding more households in over time.

Yeah, if you're playing with NRAAS's Story Progression, then by default immigration (and emigration) is turned off. You have to go in and turn it on (and you'll maybe want to set some immigrant parameters to match your preferences, although the default settings are not bad) if you want SP to generate families to live in the houses. Also, the houses have to have a fridge and a bed of some sort to be able to move a Sim/family into a house. (The type(s) of beds dictate who can move in. If you want partnered Sims to be generated, they'll need a double bed. If you want single adults, teens, and children to generate, they'll need a single bed. If you want toddlers to generate, they need a crib.) Also, the lot has to be designated a regular residential lot, not the ownable type that is, primarily, for vacation houses. If you don't turn on immigration and/or if the houses in your world don't meet minimum requirements then, yes, you will need to manually create/place families to live in the houses in your world.

If you're using EA Story Progression, I think the game will move in families over time and the only requirement is a fridge on the lot. I think. It's been ages since I've used EA's crappy story progression.