Usual disclaimer I’m not a lawyer so if it maters ect ect ect…
Copyright infringement is not always obvious as some IP already have a seated fan base doing fan art, Star Wars for example.
But
The power of the copyright is not based on facts of law but on the willingness of the copyright holder to enforce their rights. To allow fair use of IP is at the discretion of the holder so doing fan stuff with Star Wars was fine with George but not so much with Disney who now owns the property.
P.S. Doing something for free is worst than stealing the works as it debases the $ value of the IP licensing.