How do different platforms/services (not banks) make their payment systems?

Question

Payment systems

Can you tell me how different platforms / services (like GetResponse for example) make their payment systems? That is, not like with a connected payment system: when you choose a product/service on the website, and you are redirected to the website of the connected merchant (Yandex.kassa, etc.). How is this implemented? Who is the mediator? What "moves" need to be done?

Answer 1

Expensive. For a long time.
You write your billing.
You write your payment gateway.
You go through PCI DSS (some have gone through it for over a year and it cost them more than 1 million evergreens).
You conclude an agreement with a bank/international payment system.
You are connected to the processing of the bank.
Register with the relevant regulatory authorities.
The procedure is not strictly as in the answer.

Answer 2

if we are talking about accepting payment without a redirect, then you just need a service that allows you to do this
through the same iframe, you can do
it for which country you need? what currencies?