The APIs are expected to be REST APIs or OData APIs or Webservices while data exchange formats are JSON, SOAP, OAGIS, and more. Newer versions of some COTS products are supporting REST or OData APIs OOTB. However, even to access data from a single COTS system, the business needs necessitates traversal of multiple objects. Its indeed a tough task to achieve in practice with an OOTB API... https://www.1eq.com/what-we-do/common-data-model-api-factory