A purchase order starts from a supplier. You filter that supplier's products down to the ones that need reordering, type the quantities, and a running total shows the cost and what is already on order. When the delivery arrives you receive it line by line and flag anything short. The Sell Through report tells you which lines are running low or sitting dead before you start.
Yes. Each product has a supplier, and a new purchase order lists only that supplier's products, so every supplier gets their own order. You can create suppliers and categories from the product screen as you go, and bulk-edit the supplier across a filtered list of products.