code for hiding of the 'Title' field and for copying the 'Alternative text' to the 'title' attribute

use Drupal\Core\Form\FormStateInterface;
/**
 * Implements hook_form_BASE_FORM_ID_alter() for node forms.
 */
function custom_module_form_node_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  $node = $form_state->getFormObject()->getEntity();
  if ($node->getType() != 'article' && $node->getType() != 'bookmark') {
    return;
  }
  #hide single field field_image title
  $form['field_image']['widget'][0]['#title_field'] = FALSE;
  array_unshift($form['actions']['submit']['#submit'], 'custom_module_form_submit');
  #hide multiple field field_image_gallery
  for ($i = 0; $i <= $form['field_image_gallery']['widget']['#file_upload_delta']; $i++) {
    $form['field_image_gallery']['widget'][$i]['#title_field'] = FALSE;
  }
}
/**
 * Custom submit handler.
 */
function custom_module_form_submit(&$form, FormStateInterface $form_state) {
  #copy image alt to title for a single field_image
  $image_value = $form_state->getValue('field_image');
  $image_value[0]['title'] = $image_value[0]['alt'];
  $form_state->setValue('field_image', $image_value);
  #copy image alt to title for a multiple field field_image_gallery
  $field_value = $form_state->getValue('field_image_gallery');
  foreach ($field_value as &$item) {
    $item['title'] = $item['alt'];
  }
  $form_state->setValue('field_image_gallery', $field_value);
}