How can a derived class invoke private method of base class?

Because you defined the main method in PrivateOverride class. If you put the main method in Derived class, it would not compile, because .f() would not be visible there.

po.f() call in PrivateOverride class is not a polymorphism, because the f() in PrivateOverride class is private, so f() in Derived class is not overriden.