In this link here
they explain how to use junction tables to get around the many-many relationship problem in relational database design.
This line
If there is no association class, the junction table (sometimes also called a join table or linking table) will contain only the FK attributes from each side of the association.
From the example it seems that the junction table is containing not the FK but the primary keys of the individual tables.
Cust ID, Order Date
together form the composite primary key of orders and UPC is the primary key of Products
.