How to show tags + posts for a custom taxonomy?

To list names and descriptions you can use get_terms(). That will return an array with all terms. An example would be:

global $post;    
$terms = get_terms( array(
    'taxonomy' => 'item_tags',
    'hide_empty' => false,
) );
if ( ! empty( $terms ) && ! is_wp_error( $terms ) ){
    foreach ( $terms as $term ) {
        echo '<h2>' . $term->name . '</h2>';
        echo apply_filters( 'the_content', $term->description );
        $items = get_posts( array (
            'post_type' => 'item',
            'tax_query' => array(
                array(
                    'taxonomy' => 'item_tags',
                    'terms'    => $term->term_id,
                ),
            ),
        ) );
        foreach ( $items as $post ) {
            setup_postdata( $post );
            ?>
            <div>
                <h2><a href="https://wordpress.stackexchange.com/questions/302807/<?php the_permalink(); ?>"><?php the_title(); ?></a></h2>
                <?php the_excerpt(); ?>
            </div>
            <?php
        }
    }
    wp_reset_postdata();
}

Hope it helps!