When an order is completed at checkout, the customer is transferred to the Order Details page where you, the merchant get to say Thank You!
This page is customizable by you. Some simple examples of what merchants put on this page are things like a map, and when the order has a tracking number, stuff like showing where the package is. In other words, this is a good place to welcome the customer to their thing. You have access to some of the order details, allowing you to place a link there that might say "Hey! Amigo! We love you and thank you for order #1234. You can click here and take full advantage right now!"
The most basic stuff about this is in fact clearly explained and defined by Shopify in their documents. Look for info about the Order Status page. For example:
https://help.shopify.com/en/manual/orders/status-tracking/order-status-javascript-asset
Following the instructions, I pasted this simple snippet of code into the Additional Scripts and it worked perfect:
<script>
var orderId = {{ order.id }};
Shopify.Checkout.OrderStatus.addContentBox(
'<h2>Everybody do the fizzbuzz</h2>',
'<p>Order was order was was was ' + orderId + '</p>'
)
</script>