Fine-tuning an existing model - search terms and basics

I have a .blend file with a generic person and want to create a unique model from it. I don’t even know what to search for, so please help clarify. It may be specific to what I downloaded I understand.

Animation - there are bones and bone groups for hair and face, but the legs seem to be controlled by moving the feet around, and blender figures out the pose somehow. It seems to be called ‘armature’, is there a way for me to edit those relationships? And there are no bones to loc/rot/scale, is that just an option of how you set up the rig or is that just how armature works?

Modeling/Layout/Animation - these all have object/edit/pose mode, which look like they do the same thing? Just the tab determines a pre-set combination of windows? Like, Sculpting has an edit mode which operates on vertices and of course no pose mode, so how do I know which object/edit are the same and which different?

In general, I can’t find any actual bones, like I’d like to make the thigh longer, or make the legs overall longer or shorter, variation like people might have. Would you expect to find something like that, or is that the mesh/armature interaction? Like, it seems to have bones, but I can’t find them. Sculpt/weight paint should have some way to figure out what the weight applies to, but I have no idea.

Just point me to the right words to search for would be helpful, anything beyond that much appreciated.

simplified question:

Ive seen things like this, the kinematic stretch on animation seems to affect pose mode and not the base model. It also does not explain where the bones go.

I don’t think rigify was used for what I got, but the general idea should be the same I assume.