Thank you for your query.
The Post/Get HTTP codes are standard, all of them can be found here: https://developer.mozilla.org/en-US/docs/Web/HTTP/Status
Code 200 means "ok", The request succeeded. The result meaning of "success" depends on the HTTP method, more information in the link above.
Can I check if SMS was sent correctly, or only information that sending was registered in system?
Unfortunately, you can only check the response code, therefore, it shows whether the information sending was registered in the system.
Are these 6 URLs the only methods available?
Yes, these are the only URLs currently available.