Refactoring code considering differences between List<? extends>, List<? super>, and specific types

It is necessary to modify the existing code, considering the following questions:

• How do lists list1, list2, and list3 differ?
• What elements can be added to each of these lists?
• What types of objects can be safely retrieved from them?
• In which situations should you use the extends constraint, and in which — super?

class A {}
class B extends A {}
class C extends B {}

List<? extends B> list1 = ...
List<? super B> list2 = ...
List<B> list3 = ...