Access a 'global: custom text' field in fieldpluginbase render()
As indicated by @4k4 the solution is using advancedRender() inside a for loop

public function render(ResultRow $values) {
  $items = [];
  foreach ($this-&gt;options['field_types'] as $key =&gt; $value) {
   if ($value != '0') {
    $items[] = $this-&gt;view-&gt;field[$key]-&gt;advancedRender($values);
   }
  }
$build['theme_element'] = [
  '#theme' =&gt; 'dropdownlist-theme',
  '#title' =&gt; $this-&gt;t('Dropdown List'),
  '#items' =&gt; $items,
 ];
 return $build;
}