Sorry for inconvenience caused and you are looking for multi network feature without plugin so you need to do some stuff on code to access sub sites:
// Multisite
define( ‘MULTISITE’, true );
define( ‘SUBDOMAIN_INSTALL’, false );
define( ‘PATH_CURRENT_SITE’, “https://wordpress.stackexchange.com/” );
define( ‘DOMAIN_CURRENT_SITE’, $_SERVER[‘HTTP_HOST’] );
// Likely not needed anymore (your config may vary)
//define( ‘SITE_ID_CURRENT_SITE’, 1 );
//define( ‘BLOG_ID_CURRENT_SITE’, 1 );
// Uncomment and change to a URL to funnel no-site-found requests to
//define( ‘NOBLOGREDIRECT’, ‘/404/’ );
/**
* These are purposely set for maximum compliance with multisite and
* multinetwork. Your config may vary.
*/
define( ‘WP_HOME’, ‘http://’ . $_SERVER[‘HTTP_HOST’] );
define( ‘WP_SITEURL’, ‘http://’ . $_SERVER[‘HTTP_HOST’] );
For maximum flexibility, use something like this above