Plagiarism and the regulations surrounding it can be a confusing problem for authors. Journals consider plagiarism a form of scientific misconduct and their response may include rejection and long-term censorship.
If an author uses the same words from a past publication (book, journal article, website, etc.), but the copyright is held by the publisher, it is considered plagiarism, even if it they are the author of said work.
Many publishers, journals, and funding agencies routinely use software to check for plagiarism and ensure the originality of work before publication.