The threshold of originality is extremely low for a work to be considered a work of authorship. From the linked decision, " a work is “original” if it is “independently created by the author” and “possesses at least some minimal degree of creativity.” Feist Publications, Inc. v. Rural Telephone Service Co., 499 U. S. 340, 345 (1991). The lines of declaring code in the Java platform readily satisfy this “extremely low” threshold. "

Paper forms are often protected by copyright, as long as they have creative non functional elements. And they are licensed much like stock photos in some industries.

There would be some APIs or code that only contain functional elements and aren't eligible for copyright protection, but in most cases there is a substantial amount that is not only functional.

Copyright protects works fixed in a physical media, not the underlying idea. Many people can create similar works based on the same underlying ideas. Like with a map projection a specific implementation can be protected by copyright. But the idea of a map projection where a constant bearing in the real world corresponds to a straight line on a map cannot be protected by copyright, someone else could create their own version that does the same thing without infringement.

Copyright only protects against copying, not against independent creation of the same work. With something like a calendar, or in some cases an API that only has a few creative elements, multiple people could make the same choices and create the exact same thing without there being any infringement.