Any intellectual product (writing/stage shows/movies literally anything) can be plagiarized. To plagiarize 'copyrighted' material is a violation of law and can result in action in civil court.
Type parts of the answer into a seach engine box. If the answer
matches search results it is plagiarised. Sometimes people replace
words or rewrite the entire matter in their own words making it
difficult to tell whether the answer is plagiarised or not.
How do you credit a person for providing a concept in a manual?
You credit a person providing a concept in a manual by checking
that his work is original and not plagiarised at all.