As Open Terms Archive is decentralised, each instance embarks its own API. The documentation relevant to the specific version of the engine on that instance is provided on that instance itself.
The Collection API exposes JSON data over HTTP. Its OpenAPI specification can be found at http://localhost:<port>/<basePath>/<API version>/docs.
That endpoint exposes both the OpenAPI specification if the requested Content-Type is JSON, and a Swagger UI for visual and interactive documentation otherwise.
For example, the documentation of the Demo collection is publicly available for exploration.