Are you looking for the JSON file that describes the OpenAPI?
The full jar can be found here:
https://app.camunda.com/nexus/repository/camunda-bpm/org/camunda/bpm/camunda-engine-rest-openapi/7.13.0/camunda-engine-rest-openapi-7.13.0.jar
And the single file can be found here: start.camunda.com/openapi.json