As you know, API applications don't have a user interface in the browser, instead, they only display JSON or XML data, etc. Therefore, when creating an API application, the writer should create the attached documentation to support the developers who use the API, especially QA.
There are many ways to create a document, but the easiest way is to write it by hand, for example, an Excel or Word file. Specify what this API is for, what the URL is to access it, what data is sending the request, what is the response data to return, etc. If necessary, send it to the developer/QA side to use or read.
This method is very manual and labor-intensive, but it isn't always of great value to those who use it. Because there is simply no unified format and it is easy to run out of information.