Phantom's provider API is exposed to the user through the window.ethereum object that is injected into the browser. This same provider is made available at window.phantom.ethereum to prevent namespace collisions.
This API is how a dApp will make requests to the user; reading account data, connecting to the website, signing messages, and sending transactions will all be done through this provider object.
This provider API is specified in greater detail in EIP-1193.
This area of the documentation will contain information about the API's properties, events, and methods.