I want to select multiple items with various quantities

If i want to select multiple items with various quantities how would I build that?

You need to assign values to your multiple selection field https://www.jotform.com/help/301-How-to-Assign-Calculation-Value.

Once there's a value, you can add that to your form calculation widget and it should hold whatever value you assigned your multiple selection field.

Then the process in this guide https://www.jotform.com/help/259-How-to-Perform-Form-Calculation-Using-a-Widget should apply.

Let us know if you have any questions or if you need further assistance.