How do I find out which what fields are available?
Might depend a little on what you're trying to do with those fields (get info/set info/adjust info), but here are a couple of things you could try:

CustomHandler.php

For submission data:

/**
 * {@inheritdoc}
 */
public function postSave(WebformSubmissionInterface $webform_submission, $update = TRUE) {

  // Get the submitted webform data.
  $data = $webform_submission-&gt;getData();

  // Get specific submission value.
  $myFieldValue = $data['field_name']
}


For webform field values (e.g. type = radios):

/**
 * {@inheritdoc}
 */
public function postSave(WebformSubmissionInterface $webform_submission, $update = TRUE) {

  // Get the webform fields (which will contain properties/attributes/values/etc.).
  $elements = $webform_submission-&gt;getWebform()-&gt;getElementsDecoded();

  // Get field values.
  $fieldType = $elements['field_name']['#type'];
  $fieldTitle = $elements['field_name']['#title'];
  $fieldFirstOption = $elements['field_name']['#options'];['option_one'] // If you need the selected value that will be in -&gt;getData(), or in ['#return_value'] key.

  // Get webform owner (not anonymous).
  $webformOwner = $webform_submission-&gt;getWebform()-&gt;getOwner();

  // Get owner values.
  $displayName = $webformOwner-&gt;getDisplayName();
  $userName = $webformOwner-&gt;getUsername();
  $userEmail = $webformOwner-&gt;getEmail();

  // And to set data values:
  $data = $webform_submission-&gt;getData();
  $data['myNewValue'] = $myNewValue;
  $webform_submission-&gt;setData($data);

  // If you need to find some other available methods, try:
  kint($webform_submission-&gt;getWebform());
}


Fields on the webform, e.g. feedback_type, your_email, first_name will be available inside the method -&gt;getElementsDecoded().