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Built-in classes can be extended by multiple classes through the concept of inheritance. Inheritance allows a new class to be based on an existing class and inherit all the properties and methods of the parent class. To extend a built-in class, a new class is created that inherits from the built-in class using the "class" keyword and specifying the parent class in parentheses. Multiple classes can extend the same built-in class by inheriting from it in the same way.

For example, to extend the built-in "list" class, a new class can be created that inherits from it:

class MyList(list):
    def custom_method(self):
        # code for custom method

In this example, the new class "MyList" inherits from the built-in "list" class and adds a custom method. Other classes can also extend the "list" class in the same way by inheriting from it.