Define API spec from Drupal source code

Looking into the Hellosign module base, I found a single call that seems to define a set of endpoints for managing Hellosign documents, seeming to define create and access, but no deletion or update endpoint/callbacks. They are referenced in the documentation but I don't know how the hooks for this work. From what I can tell, the endpoints look something like this:

POST /admin/reports/emo/documents/hellosign/request
Body sample:

{
  "signers": [
    "required",
  ],
  "template_id": "required",
  "subject": "optional",
  "message": "optional",
}

GET /admin/reports/emo/documents/hellosign/request/{id}

Additionally, there is a callback URL set for hellosign that I can't figure out the endpoint for, or the full body definition. I think I'll need help defining the spec and requirements of this project since my Drupal knowledge is very limited and this API's spec isn't properly documented in the API docs.