Your reference is wrong, it should be a multiple to multiple association. Because each customer can have 1 to many products in it's list while each product can be a member of multiple customer lists.
The key lies in creating the correct associations in your data model. Every product should be associated to a customer. Be sure to show the product over this association so a customer only sees his own products.
This is pretty basic functionality for which you won't need a microflow. If my explanation is not clear, I would check out the online course material. https://gettingstarted.mendixcloud.com/index3.html
The solution is to model your data differently. See, for example this domain model:
The owner of the application manages the TemplateProducts, these are the base version of your products.
When you create a new customer, you initilize a subset of the TemplateProducts as CustomizedProduct. You do this by creating a CustomizedProduct object for each TemplateProduct in the subset.
You then give the customer control over his CustomizedProducts.
Now, when the customer changes a CustomizedProduct, only his own product changes.