Copyright works on derivative rules - is the component of the work unmistakenly derived from another copyrighted work.

Under at least EU AI Act, any work done by AI is not granted copyright. But it does not mean copyright does not apply, it means the amount of work credited to AI is set at 0% (simplification). A human working off another's work unless it's perfect copy will have "credit" for changes that are judged creative/transformative, meaning a human plagiarizing something still can claim to have some degree of authorship. An AI won't.

In a sense, the copyright status of final work is a sort of "sum with dilution" were each work involved adds to claims, but AI's output is set at 0 - the prompt or further rework by human is not.

As for employer, details vary but generally "work for hire" rules and contracts do reassignment of material rights (in EU and some other places you can not reassign moral rights which are a different thing).