I try to add an “alt =” attribute to the <a> of the menu in wordpress but I don’t see the form, look in the whole template and I can’t find information other than wp_nav_menu ()

<?php
                wp_nav_menu(array(
                    'theme_location' => 'primary',
                    'container' => false,
                    'fallback_cb' => 'orbital_default_menu',
                    'items_wrap' => '<ul>%3$s</ul>', ));
                    ?>

What I can hardly understand is that the <ul>%3$s</ul> variable generates <li> <a> </a> </li>

Any way to modify the <a> </a> using wp_nav_menu () ??