It is necessary to learn the API to gain more insight into how it is structured. This includes reading developer docs, making hundreds of requests with different request methods to a single endpoint and observing how it responds, and learning roles (user roles) if any that may be implemented, and understanding scopes related to the access token.
A developer's documentation gives a great insight into any API. You can learn about API endpoints which are already available publicly for use. One can understand structure, data-types, permissions, and types of request methods, which are accepted by the endpoint.
As an example shown below, Facebook's Graph API documentation gives a great understanding about any endpoint. We are looking at documentation of a user endpoint. Developer docs give you an idea about which request methods are accepted by the endpoint and what type of data needs to be sent at that endpoint.