How does adding a product after an order work?

Question

PHP

I didn’t know how to call the question)
there is a landing page baellerry.hit-sell.ru , after entering data in the order form, a person gets to the page baellerry.hit-sell.ru/finish/index.html?id=3505783
with this I have no questions.
But how do the "add to order" buttons work? How to implement it for yourself?

Answer 1

Help!