An app builder just added a lookup field to Account from the existing custom object,...

Upon adding a lookup field to Account from a custom object, Box:

D. Accounts with Boxes. Salesforce automatically creates a new report type that reflects the relationship from the perspective of the parent object, in this case, Accounts, showing related Box records.

This new report type enables users to create reports that list Accounts along with their associated Boxes, if any.

For further understanding of report types and relationships, Salesforce's guide on Report Types provides comprehensive information on how these are structured following modifications to object relationships.