The following diagram illustrates the platform integration architecture and integration points:
The API that is mostly used for platform integrations is of course the REST API. Sometimes the invocation of the REST API is hidden, such as when working with a Java client using the OpenCMIS Java library. Other times we might build on the Aikau development framework, which provides content management widgets and services so we don't have to start from scratch. But even Aikau calls the REST API under the hood.