Hi Brian,
You are just setting the associations in the ‘Change’ activity and not creating it. You have to always create associations in your domain model but need to set an association in your microflow. E.g. EntityA is associated to EntityB (one to many), this is what you defined in your domain model but when you create the object in a microflow you have to set the association either in ‘Create’ or ‘Change’ activity.
In your learning path you are creating association (Product_ProductExcelDocument) in 7.3.2 section 2 and in section 13 you are just retrieving all the products that already exist in the database and use ‘Change’ activity in the loop to set the association to ProductExcelDocument.
Hope this helps!