If you want to modify the nav menu structure or make custom menu structure in WordPress then add this code to functions.php file.

class Nav_Walker_Nav_Menu_sitemap extends Walker_Nav_Menu{
  function start_el(&$output, $item, $depth = 0, $args = array(), $id = 0){
     global $wp_query;
     $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';
     $class_names = $value = '';
     $classes = empty( $item->classes ) ? array() : (array) $item->classes;
     $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
     $class_names = ' class="'. esc_attr( $class_names ) . '"';
     $output .= $indent . '
';
     $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
     $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
     $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
     $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';
     $description  = ! empty( $item->description ) ? esc_attr( $item->description ) : '';
     $item_output = $args->before;
     $item_output .= '';
     $item_output .=$args->link_before .apply_filters( 'the_title', $item->title, $item->ID );
     $item_output .= ' ';
     $item_output .= $description.$args->link_after;
     $item_output .= '';
     $item_output .= $args->after;
       $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
    }
}

then declare the menu with new class like below example in your theme page.

wp_nav_menu( array(
    'theme_location' => 'sitemapmainpart',
    'walker' => new Nav_Walker_Nav_Menu_sitemap() 
 ) );

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