First you need to pass in the namespace to register_rest_route
Like this
add_action( 'rest_api_init', function () {
register_rest_route( 'namespace/v1', '/product/(?P<id>\d+)', array(
'methods' => 'GET',
'callback' => 'my_awesome_func',
) );
} ); Your name space namespace/v1 and your route is /product/{id} like this /namespace/v1/product/81838240219
and now you can use the route inside your function like this
function my_awesome_func( $data ) {
$product_ID = $data['id'];
} If you need to add options for ex. /namespace/v1/product/81838240219?name=Rob
and use it inside the function like this
function my_awesome_func( $data ) {
$product_ID = $data['id'];
$name = $data->get_param( 'name' );
} 