In The Observer Design Pattern How Is The Subject Class Stored?

I've been reading up on the Observer Design Pattern.

Take the below code from the following article: https://sourcemaking.com/design_patterns/observer/python/1

I understand how the code works and how the two classes, subject and observer, interrelate. What I can't quite get my head round is how this would all work in reality.

In the real world examples of the observer pattern I have read about, the interaction between the subject and observer tends to be open ended. For example, if you subscribe to a question on Quora and get notified of answers via email, theoretically, you could receive updates indefinitely.

How then if the code below was applied to a real world scenario (like Quora) do the classes persist? Are the classes and their states stored somewhere on the server?

import abc


class Subject:
    """
    Know its observers. Any number of Observer objects may observe a
    subject.
    Send a notification to its observers when its state changes.
    """

    def __init__(self):
        self._observers = set()
        self._subject_state = None

    def attach(self, observer):
        observer._subject = self
        self._observers.add(observer)

    def detach(self, observer):
        observer._subject = None
        self._observers.discard(observer)

    def _notify(self):
        for observer in self._observers:
            observer.update(self._subject_state)

    @property
    def subject_state(self):
        return self._subject_state

    @subject_state.setter
    def subject_state(self, arg):
        self._subject_state = arg
        self._notify()


class Observer(metaclass=abc.ABCMeta):
    """
    Define an updating interface for objects that should be notified of
    changes in a subject.
    """

    def __init__(self):
        self._subject = None
        self._observer_state = None

    @abc.abstractmethod
    def update(self, arg):
        pass


class ConcreteObserver(Observer):
    """
    Implement the Observer updating interface to keep its state
    consistent with the subject's.
    Store state that should stay consistent with the subject's.
    """

    def update(self, arg):
        self._observer_state = arg
        # ...


def main():
    subject = Subject()
    concrete_observer = ConcreteObserver()
    subject.attach(concrete_observer)
    subject.subject_state = 123


if __name__ == "__main__":
    main()