how to add query_vars() in other folder..

hi guys..
i have a problem.
i made some folder (url is https://example.com/test).
and i want to use $_GET value using query_vars() this function.
i add source

function add_shop_var($public_query_vars) {
$public_query_vars[] = 'product';
return $public_query_vars;
}

like this.
if i open the https://example.com/?product=1, i can get product value useing “echo intval(get_query_var(‘product’));”.
but inside /test/ folder, i can’t.
maybe url is problem.. but i don’t know how..
please help me!!