There are different levels of API design in an application development. When you create a class, its public methods are its API. When you create a package, those publicly available classes inside that package are the API of the package. When you create a web application, those HTTP endpoints that the backend provides are the APIs of the application. And you can design them in SOAP, make them RESTful, or use GraphQL. No matter what level of API you're designing, you need to make sure it is clear, consistent, and simple to use.