Hi,
When the payment is made by the customer, the payment plugin will be notified by the payment gateway, the payment plugin will change the status of the order from created to confirmed, and HikaShop will send the order status notification email to the customer. This email will automatically include a download link for each file of the products in the order.
Also, when the customer comes back to the website, the default thank you page of HikaShop contains a link to the order details page.
If the order has been confirmed, that page will also contain a download link for each file of the products in the order.