Regarding the universal adapter — was there a single protocol for external payment gateways, or is it an abstraction level?

There is generally no single protocol for all external payment gateways, as each provider implements their own API with different request formats, authentication methods, and error handling.

A universal adapter is an abstraction layer that hides these differences and provides a unified interface for interacting with different gateways. It translates calls from a common format into specific requests to each gateway and processes responses, allowing the system to work with multiple providers without changing business logic.

Thus, the universal adapter solves the problem of protocol heterogeneity, ensuring flexibility and scalability of integrations.