if ($context['field']['field_name'] != 'field_bannerimage') return;

if (($context['items']->getName() != 'field_myfield')) {
  return;
}

// if still here, our field matches. do something

function hook_field_widget_options_buttons_form_alter(...)

function mymodule_field_widget_form_alter(&$element, DrupalCoreFormFormStateInterface $form_state, $context) {
  $field_name = $context['items']->getName();
  $field = FieldConfig::loadByName('node', 'page', $field_name);

  if ($field) {
    $element['#prefix'] = 'hello world';
  }
}