Using Drupal 8 preprocess views view field was a bit tricky so I set up these snippets that should help

In your theme’ YOURTHEME.theme

/**
 * Implements hook_preprocess_form_element().
 */
function YOURTHEME_preprocess_views_view_field(&$variables) {

  // Print all the keys to see what you have available
  print_r(array_keys($variables));

  // Name of the field
  echo $variables['field']->field;
  $variables['new_variable'] = "";

  // Do something based on the name of the field
  if ($variables['field']->field == 'field_NAME_OF_YOUR_FIELD') {

    // Modify the actual output
    $variables['output'] = "add your custom" . $variables['output'];

    // And OR add new variable that will be readable in your TWIG file
    $variables['new_variable'] =  "";

  }

}

In your themes views-view-field.html.twig

{#
/**
 * @file
 * Default theme implementation for a single field in a view.
 *
 * Available variables:
 * - view: The view that the field belongs to.
 * - field: The field handler that can process the input.
 * - row: The raw result of the database query that generated this field.
 * - output: The processed output that will normally be used.
 *
 * When fetching output from the row this construct should be used:
 * data = row[field.field_alias]
 *
 * The above will guarantee that you'll always get the correct data, regardless
 * of any changes in the aliasing that might happen if the view is modified.
 *
 * @see template_preprocess_views_view_field()
 *
 * @ingroup themeable
 */
#}

{{ output -}}

{%- if new_variable is not empty -%}
  <p>
    Do something based on the new variable
  </p>
  <p>
    {{ new_variable -}}
  </p>
{% endif %}

Cheers!