Main multisite-site on app.example.com. Also fixed `cookies` related error on child-sites login

multisite-on-app.php

<?php

/**
 * Modify multisite childs subdomain path
 * The main multisite path is on app.example.com
 * And then it's children should be on foo.example.com, bar.example.com, etc.
 * This code should be located on mu-plugins folder
 */
if ( ! function_exists( 'harisrozak_change_site_subdomain' ) ) {
	function commercioo_change_site_subdomain( $data ) {
		$data['domain'] = preg_replace( '/(.*).app.(.*)/', '$1.$2', $data['domain'] );
		return $data;
	}
}

add_filter( 'wp_normalize_site_data', 'harisrozak_change_site_subdomain', 10, 1 );

wp-config.php

<?php

/**
 * Fix error on child-site login with `Cookies` error message
 * Put this line on wp-config.php
 */
define( 'COOKIE_DOMAIN', strtolower( stripslashes( $_SERVER['HTTP_HOST'] ) ) );