What is the difference between a shallow copy and a deep copy of a nested list?

A shallow copy creates a new outer collection but keeps references to its existing elements. If those elements are lists, modifying an inner list through either outer collection affects the same inner object. A deep copy recursively copies supported nested objects while tracking references to handle shared structure and cycles. Deep copying can be expensive and is not appropriate for every resource or custom object. Decide which parts must be independent rather than copying everything automatically. Immutable inner values do not have the same mutation problem.