What is "mixing in" mean in this context? The image itself is copyrighted, or it isn't. Fair use may or may not come into play for things like replication of k

What is "mixing in" mean in this context? The image itself is copyrighted, or it isn't. Fair use may or may not come into play for things like replication of known copyrighted characters, depending on commercial use. None of this has changed from the traditional copyright process. Even 15 years ago I could find some of my old works rehosted on obscure Chinese websites, and my options for recourse were the same as they are today. Many of my own works were already derivative.