How to work with payments without API, Form a form?

Question

Payment systems

Very often I see that many sites accept payments without using payment gateways. And they form links for translation.
For example, they use Yandex money or Qiwi, or Paypal. You enter the replenishment amount on the site, you are redirected to the site of this wallet with a generated form of payment, which already contains a comment, and which is asked not to be changed. For correct payment.
So, I do not understand how such payments are formed? As payment is verified, new payment.
I will be grateful for help. Would like to figure this out.

Answer 1

Everything is done through the api in the documentation of these payments, everything is described in detail with examples for many programming languages, you are just lazy to read