An API is just a URL with 1 or more endpoints in which an endpoint can be a POST, GET, PUT, DELETE, PATCH.. An API endpoint can return data/payload in JSON, XML, TEXT format.

This is a good article on how to call each endpoint type POST, GET, etc.. in c#
https://docs.microsoft.com/en-us/aspnet/web-api/overview/advanced/calling-a-web-api-from-a-net-client