adding more "npcs" ?

I have done what you said- put in premade families that I either made myself or downloaded, then evicted them. That makes them become townies/NPCs (to an extent). Every time I open up my game, I go in and delete any auto generated townies, aside from my butler.

They walk around town, populate community lots, fill positions like bartenders, waiters, baristas, delivery people, and DJs. Their pets will even wander around town like auto generated strays normally would. There are some positions the game still generates new NPCs for though. The ones I've noticed are butlers, maids, repair people, and mail carriers.

The Sims I don't tend to see walking around or filling positions much are actually the ones I've put in houses.

Hi @AJC20178 seeing NPCs out in the world doesn't have any relation to whether they live in a house AFAIK.

If you are worried about them being culled from the game, you can click the heart in the Household Manager to add them to My Households and that should preserve them.

There are mechanics that make it more likely for certain sims to show up in certain places. For example, if they have the Active trait or a Like for fitness, they are more likely to show up at the gym. Occults of all kinds are more likely at the Moonwood Mill bar. Dog owners tend to walk their dogs in certain Brindleton Bay neighborhoods. There's some kind of hidden "barfly" trait that affects whether they show up at bars, not sure how that one works. Etc.
You can manipulate this kind of thing directly if you have Get Together, by making clubs that hang out at particular lots.

When I'm feeling bored with my gameplay one of the things that helps is finding townies or randomly generated sims and opening them up in cas. Sometimes I randomize, sometimes I make a specifically themed person.