"Stupid Question": I want to know what's that called and how to "fix it"

As I wanted to separate the meshes to add different “texture decals” on the wall, I’ve encountered this…
What’s the name of it and why when in Edit Mode selects two vertices from the “submesh” instead of one?

Is this some kind of “advanced mesh parenting”?

These objects are duplicated “Linked”
https://docs.blender.org/manual/en/latest/scene_layout/object/editing/duplication.html#linked-duplicates

Blender still thinks of them as the same Object so any edits you do to one will apply to all of them.

You are trying to duplicate “unlinked”
https://docs.blender.org/manual/en/latest/scene_layout/object/editing/duplication.html#duplicate

This will create a new object disconnected from the original one.

However you can also create a new UV map to apply two textures on the same surface.
image
image

image

This kind of thing…