//If you want to get Product id from order id in Woocommerce then fetch order details.

$order = new WC_Order( $order_id );
$items = $order->get_items();
//then if you loop through them, you can get all the relevant data:

foreach ( $items as $item ) {
$product_name = $item['name'];
$product_id = $item['product_id'];
$product_variation_id = $item['variation_id'];
}