WordPress Walker Classes for dropdown menu

To achieve the desired menu structure, you can create a custom WordPress Walker. The Walker class will extend the Walker_Nav_Menu class, and you can override its methods to output the HTML structure you need. Below is an example of a custom WordPress Walker for your specified menu format:

class Custom_Nav_Walker extends Walker_Nav_Menu {
    function start_lvl( &$output, $depth = 0, $args = null ) {
        $indent = str_repeat("\t", $depth);
        $output .= "\n$indent<ul class=\"dropdown-menu\">\n";
    }

    function start_el( &$output, $item, $depth = 0, $args = null ) {
        $indent = ( $depth ) ? str_repeat("\t", $depth) : '';

        $class_names = $value = '';

        $classes = empty( $item->classes ) ? array() : (array) $item->classes;
        $classes[] = 'nav-item';

        $class_names = join(' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );
        $class_names = ' class="' . esc_attr( $class_names ) . '"';

        $id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
        $id = strlen( $id ) ? ' id="' . esc_attr( $id ) . '"' : '';

        $output .= $indent . '<li' . $id . $value . $class_names .'>';

        $atts = array();
        $atts['title']  = ! empty( $item->attr_title ) ? esc_attr( $item->attr_title ) : '';
        $atts['target'] = ! empty( $item->target )     ? esc_attr( $item->target     ) : '';
        $atts['rel']    = ! empty( $item->xfn )        ? esc_attr( $item->xfn        ) : '';
        $atts['href']   = ! empty( $item->url )        ? esc_attr( $item->url        ) : '';

        $atts = apply_filters( 'nav_menu_link_attributes', $atts, $item, $args );

        $attributes = '';
        foreach ( $atts as $attr => $value ) {
            if ( ! empty( $value ) ) {
                $value = ('href' === $attr) ? esc_url( $value ) : esc_attr( $value );
                $attributes .= ' ' . $attr . '="' . $value . '"';
            }
        }

        $item_output = $args->before;
        $item_output .= '<a'. $attributes .' class="nav-link">';
        $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
        $item_output .= '</a>';
        $item_output .= $args->after;

        $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
    }
}

// Usage in your theme's menu output
wp_nav_menu(array(
    'theme_location' => 'your_menu_location',
    'walker' => new Custom_Nav_Walker(),
    'menu_class' => 'navbar-nav mx-auto style2'
));