Backend API Workflow with Groupby

When a button is clicked, I need to schedule a backend API workflow to create a list of new things grouped by month from a data type named Inventory to a different data type named Monthly Report.
I can do this in a repeating group using the Inventory Data Type and groupby, but I can’t figure out how to do this from one data type to another?

Please help… I’ve been trying to figure this one out for days…