Add custom fields to core modules in Joomla! 3

add-custom-fields-to-core-modules-j3x.php

<?php
defined('_JEXEC') or die;
?>
<div class="latestnews">
   <ul class="list-group list-group-flush">
      <?php foreach ($list as $item) : $image = json_decode($item->images); ?>
      <?php
        // most important part
        // context: 
        // for articles: com_content.article
        // for users: com_users.user
        // etc...
        $jcfields = FieldsHelper::getFields('com_content.article', $item, true);
        
        // bonus: custom fields by name
        // usage: $fields_by_name['name-of-field']->value
        $fields_by_name = \Joomla\Utilities\ArrayHelper::pivot($jcfields, 'name');
        
      ?>
      <li itemscope itemtype="https://schema.org/Article" class="list-group-item px-0" style="background-color:<?php echo $fields_by_name['article-favourite-color']->value; ?>">
         <?php if($image->image_intro != "") : ?>
            <a class="article-media" href="<?php echo $item->link; ?>" itemprop="url">
               <img class="card-img-top pb-2" src="<?php echo JURI::root().$image->image_intro; ?>" alt="<?php echo htmlspecialchars($image->image_fulltext_alt); ?>">
            </a>
         <?php endif; ?>
         <a class="article-title" href="<?php echo $item->link; ?>" itemprop="url">
            <span itemprop="name">
               <?php echo $item->title; ?>
            </span>
         </a>
      </li>
      <?php endforeach; ?>
   </ul>
</div>

	// custom fields by ID
	// usage: $fields_by_id[X]->value, X being the ID of the custom field
	$fields_by_id = \Joomla\Utilities\ArrayHelper::pivot($jcfields, 'id');

Example of real use for a custom field having ID 28 and Name 'institution'

My custom field with $fields_by_id: value; ?>

My custom field with $fields_by_name: value; ?>