People sell all kinds of things that are based on things in the public domain, because people are paying for someone else to go through the steps of creating it. Creating new works based on something in the public domain, like a 3D model of a mouse that is in the public domain, is a new work that can be copyrighted.

This person could sell the 3D files they created because creating the 3D files is a new creation. Someone else could also create the exact same 3D file from scratch and this person could not claim copyright because the subject of the 3D file is in the public domain.

Anyone can copy the Disney film version outright because the film is in the public domain.

That's not how it works. If I make a new work based on something in the public domain, your work is copyrighted. No one can copy your 3D model, unless your license allows it.

Only the original is public domain, what you do based on it isn't.

To keep the example Disney, Alice in Wonderland, the book, is public domain. So is One Thousand and One Nights, the story Aladdin makes use of. The films Disney made adapting them are not.
You can create your own adaptation of the books, or even just reprint them as is, but you can't resell or modify the Disney versions.