The typical initial setup would be
One PO has many Line items typical;ly written like PO < Line Items
One Item Master SKU has Many Line Items Item master SKU < Line Items
ie the wide part of the < points to the many side of the relationship.
so that is the typical many to many join that for the line items which will have the qty and the price.
Then it sounds like One PO has many Shipments
One Shipment has many Attached Part Assignments
One PO Line Item has many Attached Part Assignments.
SO Attached part Assignments are akin to the "line items" on the shipments.
The drop down list for the Attached Part Assignments will be conditional on first selecting a [Related Purchase Order]. That [Related Purchase Order] will be a lookup from the Shipments table.