Yes ^ ^ ^ the above… but there are limitations. Generally you cannot ADD new items, only change existing items. Here’s why:
Any arbitrary FBX model (including Blender files automatically imported as FBX files in Unity) is a collection of things:
- each object becomes a GameObject
- each might have a mesh
- each might generate / use materials
- there are parent/child relations between objects
The collective bunch of things is imported and given Unity-side names (called GUIDs) based on their Blender / FBX names and relationships.
When you drag it into Unity and make a prefab, a Prefab is just a collection of stuff linked together.
If you change a mesh in the Blender file the Prefab will just start using it.
However if you change the “Shape” of your hierarchy, that changes the internal naming, breaking your prefab.
Experiment!