An API (Application Programming Interface) is a program which allows public
access into a specific system.
1. Internally it is a collection of methods (public and private) which is written for a specific environment (language, operating system, and other infrastructure).
2. Externally, accessing the API via an endpoint
point; it is by nature agnostic to what is accessing it- as long as the format of the request is correct and complete.
The best way for you to learn how to create an API is probably to follow a tutorial or two; such as these for WebAPI:
1. Create Simple Web API In ASP.NET MVC
2. Tutorial: Create a web API with ASP.NET Core | Microsoft Docs
The API clients can now access the functions of this by creating a standardized request. The API Request itself is (programming) language agnostic, as long as the Request that is made
is the same it does not matter if I created the request by typing it into a browser, or if I used SoapUI, or I accessed it within another application by making an HTTP request.
While the Request is language agnostic, making a request via code is going to depend on whatever language you are writing it in.
Python code does not run in a C# environment; BUT, there is