I usually have a backstory in my head before making a sim/family or even save game with numerous families. It can be original (as much as things like that are really original) or it can be inspired by a character or cast of characters I've run across through books, tv, movies, other games including past sims games or even other sources. The characters can be nearly any age but are usually young adults. These also have to be characters that I think I can actually play out in the sims game or at least be okay with how I'll actually live out their lives. Mods sometimes come into play also because some things I never expect to see in the sims mostly due to ratings so if a mod has it and it works well enough to suit me I can tell more stories with those.
My go to example for the forums is I have a Stardew Valley save. I've created nearly every character that appears in that game and they all have a life that is inspired by that game but conformed to what is possible in this game. So I'll give them jobs as close to what I think they might have. I'll dress them as I think they would dress. I'll look for or build or tweak houses that I see them residing in. I'll work toward the relationships I see them having and the goals they might (or might not) have.
Then I play in other ways. Sometimes I just play deviantly for a short session, sometimes I want to try out a new aspiration or something else from a new pack or even an old pack that I never got around to trying out so I make a character just to do that. I also sometimes try to do player made challenges or the ones I come up with or bend the rules for to guild my own play a bit. Sometimes I just want to test out how many ghost types I can make... or other things like that to amuse me and see what's possible.
Lately I took inspiration from the newest premades in IL... and now I have a rotational save game that is just about the all the premades that came with game.
I don't have a shortage of ways to play.