I write my own papers, but will put paragraphs through an llm and ask it how it can be improved (normally grammarly's 'ai'), and sometimes I take it's advice, but half the time I dislike what it's done. Sometimes I give it a bunch of information on what I need to write, and it'll spit something out, and then I'll sort of use it as a skeleton for my paper, but to be honest, it's kind of shit, regardless of which one I've tried. And it lies. So much.

Replace LLM with a person. If it was a person editing your work, does it make it plagiarism?

A common proofreading technique is to give your work to another person to read and make comments. That's not plagiarism.