field-paragraph-condation

.module

/**
 * Implements hook_field_widget_WIDGET_TYPE_form_alter().
 */
function my_common_field_widget_entity_reference_paragraphs_form_alter(&$element, \Drupal\Core\Form\FormStateInterface $form_state, $context) {
  /** @var \Drupal\field\Entity\FieldConfig $field_definition */
  
  $field_definition = $context['items']->getFieldDefinition();

  $paragraph_entity_reference_field_name = $field_definition->getName();

  if ($paragraph_entity_reference_field_name == 'field_paragarph') {
    /** @see \Drupal\paragraphs\Plugin\Field\FieldWidget\ParagraphsWidget::formElement() */
    $widget_state = \Drupal\Core\Field\WidgetBase::getWidgetState($element['#field_parents'], $paragraph_entity_reference_field_name, $form_state);

    /** @var \Drupal\paragraphs\Entity\Paragraph $paragraph */
    $paragraph_instance = $widget_state['paragraphs'][$element['#delta']]['entity'];
    $paragraph_type = $paragraph_instance->bundle();

    // Determine which paragraph type is being embedded.
    if ($paragraph_type == 'test_form') {

      $dependee_field_name = 'field_select_type';

      $selector = sprintf('select[name="%s[%d][subform][%s]"]', $paragraph_entity_reference_field_name, $element['#delta'], $dependee_field_name);

      // Dependent fields.
      $element['subform']['field_upload']['#states'] = [
        'visible' => [
          $selector => ['value' => 'upload'],
       ],
      ];

      $element['subform']['field_youtube']['#states'] = [
        'visible' => [
          $selector => ['value' => 'youtube'],
        ],
      ];
    }
  }
}

Always remember
Good point above. To be specific, considering the hook name hook_field_widget_WIDGET_TYPE_form_alter:

If you're using the "Paragraphs Classic" widget: WIDGET_TYPE = entity_reference_paragraphs
If you're using the "Paragraphs Experimental" widget: WIDGET_TYPE = paragraphs