Guide on how to use moodle existing functions in a custom webservice plugin

Hello, I'm new at moodle system development. Right now, I'm working on to make a custom webservice plugin.
My question is, is there any example of how to use or integrate the existing moodle's service function (eg. create a course, bulk create users or enroll them) to a moodle webservice plugin?
Because so far what I've done is by only using moodle's dml API.