Hi, to start with a small stripped out sample family would be helpful to assist, but hope the below is what you actually need…
If you want to select a nested family within a parent family, you can use “Element Dependents” component to select them.
From that nested family…there is element references component that can get geometric references within the element which you can use to create geometry if that’s what you are after… you can refer to below post and go through it might be helpful.
Let us know if that works out for you…