Adding a secondary menu to my theme

There’s a lot to read about secondary menus in WP, however it looks like the tricks are for older WP version.
I see that WP 4.5 supports multiple menus in the dashboard, but it doesn’t show a location where to insert the menu.
Do we still need the following code in the header?

<?php wp_nav_menu( array( 'theme_location' => 'secondary-menu', 'menu_class' => 'topper', 'fallback_cb' => '') ); ?>

Moreover, Do i need to place this code in functions.php?

add_action( 'init', 'my_custom_menus' );
  function my_custom_menus() {
     register_nav_menus(
        array(
  'primary-menu' => __( 'Primary Menu' ),
  'secondary-menu' => __( 'Secondary Menu' )
                )
         );
  }

Last question is: will the secondary menu be automatically visible in the mobile menu version?