how can I make a certain model follow only the legs or arms?

[languid falcon]

Wdym “model”, is this a mesh replacement shirt or what

[languid falcon]

So mesh replacement shirt item

Well, dunno

But I can write down how to do it

Heres the vanilla mesh and rig

Add your stuff on it

And then weight paint it

For firstperson, you need to duplicate the final model, and only keep the arms