Hi lili ,
You're welcome.
If you ask for the BOM variable product, BOM products are all hidden from the front end unless you mark them as sellable.
What do you mean by this?
Is there an option to link the products easily by doing it one by one on the product page?
Here you can read how to set the products' BOMs.
On the stock central, what's the best method to manage the inventory regarding hierarchy?
You can read here all Product Level's docs.
Best Regards,
José Andrés