I am learning about copy right law, we are currently learning about copyright law pertaining to written works. Such as if an author finished the work of a previous author, what would that be considered. Furthermore, does the second author who finished the work of the first, have the right to se a character in future works?

This is essentially how I am interpreting the question below. Is anyone able to help explain this to me?

https://www.copyright.gov/

Robert B. Parker published a novel called "Poodle Springs", finishing an incomplete work by Raymond Chandler, who was long since dead. He later published two more books using the Phillip Marlowe character. How was this able to be done?