I hope this helps
I added comments to the WP_Query array below.
<?php
// query
$the_query = new WP_Query(array(
'post_type' => 'event', // (the post type / custom post type)
'posts_per_page' => -1,
'meta_key' => 'featured', // (acf-field-name-NOT-label)
'orderby' => 'meta_value', // (Don't change)
'order' => 'DESC' // (or ASC or RAND)
));
?>
<?php if( $the_query->have_posts() ): ?>
<ul>
<?php while( $the_query->have_posts() ) : $the_query->the_post();
$class = get_field('featured') ? 'class="featured"' : '';
?>
<li <?php echo $class; ?>>
<a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
</li>
<?php endwhile; ?>
</ul>
<?php endif; ?>
<?php wp_reset_query(); // Restore global post data stomped by the_post(). ?>
https://www.advancedcustomfields.com/resources/orde-posts-by-custom-fields/