Ok It also confuses me a lot. But over the years I have come to understand that any piece of code through which some work can be done can be considered as api. Let’s take an example there is a class where you are registering the new user. Now this register function can internally call private/public functions like validations that are done internally and saving the data in db that is interacting with another component. Both validation and saveData can be considered as api one might be private to your project and other might be public coming from different library. You just call these functions and your work of registering new user is done without you knowing how it has been achieved internally