Help for developing an extension for SMS
You might want to start with https://docs.civicrm.org/dev/en/latest/ if you haven't seen that before. To try out API functions, I would encourage you to login to a civi instance and go to the menu beside Administer, ie Support  &gt; Developer &gt; APIv3 (or preferrably APIv4 if it has what you want), then test things there. The same functionality is available on your iwn instance. Note especially the callback methods available under https://docs.civicrm.org/dev/en/latest/hooks/
There are CiviCRM logs available at sites/default/files/civicrm/ConfigAndLog/ but you may also need to look at your Apache and occasionally MySQL logs too. Welcome, and good luck.