Nova Custom Tools

General guidance / docs

For simplification, If I have a Nova resource called InventoryClassification with one field - name that persists to the DB and I want to create a custom tool for adding new inventory items, how am I linking the has many relationship between classifications to inventory item and belongs to relationship of inventory item to inventory classification on the custom tool?

The only class file I see for the custom tool are in src directory. Is this the file I should be defining my custom tool relationships in?