getting content from main domain to sub-domain using category and WP_Query

What you can do is to set up a pre_get_posts() filter in the subdomains’ theme functions.php file to restrict posts to the desired category…

function my_subdomain_category( $query ) 
{
    if ( $query->is_main_query() ) 
    {
        $query->set( 'cat', '123' ); // use the categoryID for space or products
    }
}
add_action( 'pre_get_posts', 'my_subdomain_category' );