You may see Unpaid Payments: $XXX for projects in project list

or Unpaid Payments count on Project page in admin panel

But how these are calculated?

In project create/edit form you will see Project Budget field.

If you set a budget for a project then you should add payments for that project using Finance -> Payments module.

Project unpaid payments are calculated by

Project Payments Received = Payments entries for the project in Payments Module

Project Budget = Value set for Project Budget field in Project add/edit form

Project Unpaid Payments  = Project Budget - Project Payments Received