You’ll need to tie into template_redirect
hook where you will check the site and the user and then redirect if needed.
function redirect_to_subsite() {
if( is_main_site() && !current_user_can('manage_network') ) {
$blog_id = 5; // <<----- Update the ID to the subsite blog ID you want to redirect to
wp_redirect( get_site_url( $blog_id ) );
exit();
}
}
add_action( 'template_redirect', 'redirect_to_subsite' );