Hi,
Perhaps this page can help you: https://docs.mendix.com/howto6/working-with-lists-in-a-microflow.
For calculation of totals: https://docs.mendix.com/howto6/working-with-lists-in-a-microflow#4-calculating-a-total-list-value-using-a-variable-and-a-loop and https://docs.mendix.com/howto6/working-with-lists-in-a-microflow#5-calculate-a-total-list-value-using-an-aggregate-function.
I think that when you read this page, you will get your answer on how to get the total amount of products per supplier.