All 15 Multiple Inheritance & MRO questions
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How does the built-in `object` class relate to every class's MRO, and why does it always appear last?

Intermediate

Answer

Every class in Python 3 implicitly inherits from `object` (directly or transitively), which provides default implementations of fundamental magic methods like __init__, __repr__, __eq__, and __hash__. Because every other class is 'more specific' than object, C3 linearization always places object last in the MRO -- it's the common ancestor that every other class's MRO must be consistent with appearing after.
class Foo:
    pass

print(Foo.__mro__)  # (<class 'Foo'>, <class 'object'>)
print(issubclass(Foo, object))  # True, implicitly
Real-world example Understanding that object always terminates the MRO clarifies why calling super().__init__() in a deeply nested class hierarchy eventually reaches object.__init__(), which does nothing but must still be part of the chain.

Common follow-ups: What default methods does object provide?;Why did Python 2 require explicit inheritance from object for 'new-style classes'?

Multiple Inheritance & MRO;Magic Methods & Operator Overloading
All 15 Multiple Inheritance & MRO questions