Plagiarism is the wrongful appropriation of someone’s ideas and passing them off as your own. For example, referencing another writer’s work without citing it, or copying on a test, or having someone write a paper or article for you and not giving them credit (ghost writing). It is ethically wrong, as it is considered in academic forums and journalistic forums a violation of academic integrity.
Copyright infringement is a legal issue, where a content creator has filed for the right to protect his/her creative works, including writing.
If an advertiser or business has not copyrighted their material, then no infringement is possible. For example, if I state “Our produce is so fresh, we get weeds in our stores,” (sorry, I’m not very creative) that’s not a copyrighted phrase. I could make it one - think of Coca-Cola’s slogans over the years, like “Add a Coke and a smile” - those are copyrighted. Even the design of Coke’s bottles is copyrighted. If you copy those, you can be sued, but that is not plagiarism unless it is in the academic and journalism fields. In publishing, most material is copyrighted.
That said, back to the OP, copying marketing materials or designs is neither copyright infringement or plagiarism, unless it is part of a class assignment. Many companies find out about what a competitor is doing and they market their product using similar key phrases, or even what sounds like a copy of the material. The content creator could always copyright the phrase or idea, which would then protect them legally.
FWIW, I’m a university professor who has had countless students cheat and plagiarize. I’ve also advised clients as a consultant to look at “best practices,” - i.e. what other companies do - and take the elements that work for their own business.
It is a grey line, but like most things in ethics, you usually know if what you’re doing would pass the sniff test.