This project is example code showing how you can use the Moodle forms API along with Javascript and AJAX in a Moodle plugin to create a dynamic dropdown list tat pulls data from moodle depending on the selction of a dropdown in a Moodle form. The demonstration plugin can be installed ona Moodel test server using the standard Moodle plugin installation process.
A code walk through is available at https://www.youtube.com/watch?v=7JuStA4WwVU
If you are looking for Moodle plugin or Moodle theme development courses. Please go to https://ricoshae.com.au/moodlecoursesgh to view the courses.