Even before getting into the copyrightability of code, at the very least, any LLM-produced parts are not copyrightable. They are public domain.

That said, if its a mix of LLM code and human code, things get pretty messy. From my understanding, if the human expanded on or modified AI code, its public domain. If they wrote a section fully independently, they absolutely own the copyright. If its an unclear mix, it would have to be proven on a case-by-case basis with the onus being on the AI user to provide solid evidence that the code copied isn't AI generated.