how crete menu in wordpress? code example

Example 1: wp main menu

wp_nav_menu( array( 'theme_location' => 'header', 'menu' => 'Main Menu') );

Example 2: add top menu bar in wordpress

function register_my_menu() {
register_nav_menu('additional-menu',__( 'Additional Menu' ));
}
add_action( 'init', 'register_my_menu' );