Purchase and Sale Order

I’m trying version 5.1.
I kindly ask for clarification regarding Sale Order and Purchase Order with the configuration INTERCO = true.
By generating a Purchase Order between two Companies, a Sale Order is generated for the supplier company (perfect).

  1. Are these two documents then no longer connected in any way? Specifically, if I complete the SO, the PO is not updated in any way (for example with a change of state)?
  2. When I close the SO movements are generated between “Main Warehouse” (INTERNAL selected in the SO) and “Customer” (VIRTUAL), while when I close the PO generates movements between the “Waste” (internal of the company supplier) and "Main Warehouse "(INTERNAL selected in the PO), why?
  3. According to which logic is the “Waste” warehouse selected? (I noticed in the database that is the first warehouse that is generated by the import of the demo data)
  4. According to this logic, the goods, from the “Main Warehouse” of the supplier is moved twice (one from the SO and one from the PO) I do not think is correct, right?

Thank you for your help in this matter.