When it comes to the actual transactions themselves, there are a few different platforms in which they take place. Most of the financial institutions out there don’t process the transactions themselves, but instead they rely on other third party companies to do the heavy lifting for them.
The three main that merchant processors rely on are: Banks, Associations, and ISOs (Independent Sales Organizations).
Here is an article on How to find the Right ISO for Your Merchants which will better educate you in making a decision that’s a fit for your business.