How to display a field of the parent store in my module?
Sounds fun, Here is an option to do this. 

1) make a controller :
routingfile:

my_module.store_terms:
  path: '/store-terms/{store_id}'
  defaults:
    _controller: '\Drupal\my_module\Controller\StoreTermsController::showTerms'
    _title: 'Terms'
  requirements:
    _permission: 'access content'


In the controller:

use Drupal\Core\Ajax\AjaxResponse;

use Drupal\Core\Ajax\OpenModalDialogCommand;

public function showTerms($store_id) {
  $store = \Drupal::entityTypeManager()-&gt;getStorage('commerce_store')-&gt;load($store_id);

  $terms = $store-&gt;field_terms_and_condition-&gt;value;

  $render_array = [
    '#markup' =&gt; $terms
  ];
  $response = new AjaxResponse();
  // Show a dialog box.
  $dialogText = $render_array;
  $dialogText['#attached']['library'][] = 'core/drupal.dialog.ajax';
  $response-&gt;addCommand(new OpenModalDialogCommand('Terms and Conditions', 
   $dialogText, ['width' =&gt; '500']));
  return $response;
}


then back in your  buildPaneForm method:

$attributes = [
  'attributes' =&gt; [
    'class' =&gt; 'use-ajax',
    ],
];
$link = Link::fromTextAndUrl(
  t('terms and conditions of the store "@store_name"', ['@store_name' =&gt; 
   $store_name]),
  Url::fromUri("internal:/store-terms/".$store_id , $attributes)
  )-&gt;toString();