I feel like you missed a bit of the explanation out there. Where does the struct like class end up being instantiated and how is it linked to the class that pre

I feel like you missed a bit of the explanation out there. Where does the struct like class end up being instantiated and how is it linked to the class that presumably would have previously contained it? Does the struct like class get put into some kind of data store and the class that needed mutable state then holds a handle or similar?