For copyright infringement to occur, the writing, photograph, song, etc. must in fact be copyrighted. Even just the addition of one's name, the date, and the symbol © is usually sufficient to create a copyright, although formal works are often submitted to the government for registration as having been copyrighted. Plagiarism need not involve a copyrighted work, but is simply the presentation of the words or organizing thoughts of another as one's own with adequate citation. A recent notable example of plagiarism is Melania Trump's use of significant passages of Michelle Obama's speech at an earlier Democratic convention without giving credit to Mrs. Obama.
Reg Shrader
Reg Shrader