It's not a class with a constructor that you can instantiate, so we don't call it a class. It's not a prototype object either.

MDN uses the term interface in the generic OOP meaning, which is not restricted to class-based inheritance but refers to a type definition with method signatures.

However, it also uses the term interface in the very specific context of the Web Interface definition language, which the web storage specification uses to define Storage as an interface indeed. These WebAPIs can be implemented in multiple languages (called "bindings"), though JS is most common. The WebIDL spec even defines how such an interface is to be represented in JavaScript (the "ECMAScript binding"), in particular that the linear inheritance of interfaces is implemented using prototype inheritance between interface objects and their .prototypes (basically as if using class Storage extends …). This means localStorage instanceof Storage and Storage.prototype.hasOwnProperty('getItem') work as expected.

You're almost right about the keydown event. It's supposed to call the callBack function with each keydown press when the input HTML element is activated or it is able receive the event (the HTML element with the #textBox id). It means that if the #textBox element is not selected/activated, it will not listen to any 'keydown' event, and as a result it will not call the callBack function: (event) => output.textContent = You pressed "${event.key}".

keydown is an event which is listened by the #textBox element via addEventListener() method. To be more clear, the addEventListener() method sets up a function that will be called whenever the specified event is delivered to the target. The target in this case is the html element with the #textBox id.

In short the code you wrote will add the keydown event to the input element, and call the (event) => output.textContent = You pressed "${event.key}". function when the event is happening.

You can read more about addEventListener() on MDN.