Comprehensive information about payment gateways

Hello, my name is Hawary, I want to start a payment gateway business in the United Arab Emirates, I want comprehensive information about the payment gateway

First, I want to know what is the payment gateway and how it works in detail

Second I want to know all the requirements for operating a payment gateway in detail

Third, I heard that I need to obtain a security certificate (Pci dss) I want to know what it is in detail and how to get it

Fourth, as an entrepreneur who wants to start payment gateway business , what are all the things he needs from hiring people to the other?

NOTE** Only those who have experience in this field can submit an offer