In Salesforce B2C Commerce, when developing a new Page Designer page type, the rendering logic should be defined using a Controller file with a specific route, typically named 'render'. This controller manages the backend logic necessary to fetch and process data before rendering the page. The 'render' route in the controller is responsible for generating the output that will be delivered to the client's browser, effectively controlling how the page will display the data. This method allows developers to create dynamic, responsive pages tailored to merchant's needs while maintaining clean separation between the application logic and presentation layers.