If you've added an api in the extension, it will be automatically seen in the API explorer, provided -
- The extension is enabled.
- The API is created as per https://docs.civicrm.org/dev/en/latest/extensions/civix/#generate-api
- The api file need to be under
/path/to/extension/api/v3/your_file.phpwhich is auto-created if you've followed the steps in the above documentation