When a company chooses to keep their products a trade secret rather than patent it, they don't have to make it public knowledge. But they also lose they protection that patents give. So, if another company or individual were to independently invent that product, they could patent it and force the original manufacturer to end production. However, to patent the product, they would truly have to invent it and produce credible documentation. They cannot reverse-engineer the product or have the secret leaked to them and pass it off as their own invention.
Copyrights protect forms of expression that are originally authored works.
However, others may make fair use of the work as long as the use follows four factors. Nature of the work Works that have previously been published are easier to use than works that are unpublished. For example, if you took two words from a song, it would probably be ok. For example, it is usually ok to make use of sections of a book, even a chapter. However, if that book is known only for one of its chapters and that is the main reason people buy it, it would be copyright infringement to make use of that whole chapter.