Use Apex interfaces if you want to completely customize the TMF621 APIs and only use the Salesforce framework for integrations. The Salesforce framework includes the consent check, permissions and licensing check, and invocation of the REST resource.
You can use the TMFOpenAPITroubleTicketService Apex interface to customize TMF621 API.
By default, no implementation is associated to the interfaces. If you want to add your own implementation of the API handlers, write a class implementing these interfaces and add the details to the interface implementation on the org.