Explanation
The API-based record creation is generally handled in the service-layer responsible for the entity in Salesforce B2B Commerce. The service-layer is a set of classes that provide methods for interacting with the data layer and performing business logic. Each entity, such as product, cart, or order, has a corresponding service class that handles the create, read, update, and delete operations for that entity. For example, ccrz.ccServiceProduct provides methods for creating and retrieving products. Salesforce References: B2B Commerce and D2C Commerce Developer Guide, Service Classes