WP taxonomy. WP_tax.

children_of_tax.php

<?php
$term_id = 10;
$taxonomy_name = 'products';
$term_children = get_term_children( $term_id, $taxonomy_name );

echo '<ul>';
foreach ( $term_children as $child ) {
	$term = get_term_by( 'id', $child, $taxonomy_name );
	echo '<li><a href="' . get_term_link( $child, $taxonomy_name ) . '">' . $term->name . '</a></li>';
}
echo '</ul>';
?> 

index.php

<?php global $post; ?>
<?php if($post_terms = get_the_terms( $post, 'taxonomy_slug' )): ?>
  <div class="subtitle"><?php echo $post_terms[0]->name; ?></div>
<?php endif; ?>

relation_terms.php

ACF relation with dinamic terms
<?php if( $post_arr): ?>
    <?php foreach( $post_arr as $post): ?>
        <?php setup_postdata($post); ?>

        <div class="post">
            <div class="post__inner">
                <a href="<?php the_permalink(); ?>" class="post-link">
                    <?php if ($view_subtitle && $header_view == 'title'): ?>
                        <div class="subtitle subtitle--gray"><?php echo $view_subtitle; ?></div>
                    <?php elseif( ($post_terms = wp_get_object_terms($post->ID, get_object_taxonomies($post->post_type))) &&  $header_view == 'cat'): ?>
                        <div class="subtitle subtitle--gray"><?php echo $post_terms[0]->name; ?></div>
                    <?php endif; ?>
                    <h2 class="post__title"><?php the_title(); ?></h2>
                    <div class="post__excerpt"><?php the_excerpt(); ?></div>
                    <span class="arrow icon-arrow-right1"></span>
                </a>
            </div>
        </div>
    <?php endforeach; ?>
    <?php wp_reset_postdata(); ?>
<?php endif; ?>

Get chirdren of taxonomy - https://codex.wordpress.org/Function_Reference/get_term_children