Very new, trying to figure out how NPC tuning works

Hello, I'm very very new to Sims 4 Studio (and forums in general, please be nice...!). I've been browsing tutorials and doing my best to Google what I'm looking for, but I can't seem to find anything relevant to what I'm trying to do. Essentially I'm trying to edit NPCs so that on a new game, they'll have traits, relationships, etc. exactly how I want them, if that makes sense. So for example, making Nancy Landgraab count as Johnny Zest's mother, have a negative relationship with him, and a negative sentiment. I know stuff like this is possible because I've seen at least one mod that edits relationships and whatnot between characters, but I'm completely lost on how to successfully do this myself.

By just browsing around in the "Export Tuning" files I've found that a lot of NPCs seem to have tuning files that affect their starting "stats" if you will, but not all NPCs, mostly ones from after the first few DLCs it seems? I've heard that many base game and early DLC NPCs don't have tuning files, but I was wondering if it's possible to create one from scratch anyway, instead of just editing an existing one. I honestly don't know a lot about how code works, I mostly just trial and error my way through stuff, but like... I guess maybe if I found Nancy Landgraab's "character ID" or something, could I create relationships, likes and dislikes, etc. for her? And if not, then how exactly does the game "know" what kinds of relationships and behaviors to give her, if not through a file like this?

Sorry if I seem ignorant or scattered, I'm just trying to learn about this stuff so I can make my own mods, and none of the tutorials or resources I can find online seem to say... well literally ANYTHING about altering NPCs. It's all just about CAS stuff, custom objects and interactions and the like. I would very much appreciate help!