A Basket is used to group a set of order lines. An order is placed for an individual item.
An Order can have multiple copies/units placed against it. A Basket can have multiple Orders.
It is highly advisable that you get in the habit of checking the catalogue first before placing an order for a new item. This saves unnecessary duplicate Bibliographic records in your database and any need to merge/deduplicate records.
No. A basket is simply a way that a set of orders can be grouped together. You may chose to "Close A Basket" when the Order for all the items in the basket has been placed with the Vendor. A basket does not need to be closed to receive the items that have been ordered within that basket.
Purchase order numbers are entered against orders, not baskets. See Add To Order