First, a few disclaimers, I am not a lawyer and this is an actively evolving area.
The answer that most aligns with current precedent to my knowledge is that the parts you modify are protected by your copyright, but the rest remains uncopyrightable. With the exception of any chunks generated that align with someone's existing copyrighted code, as long as those chunks are substantial and unique enough.
This is how I understand it as well. There was a case of a monkey taking a selfie and the photog attempting to assert copyright. Courts determined (if I remember correctly :p ) that only human works are copyrightable and therefore the monkey's selfie could not be copyrighted at all.
The answer that most aligns with current precedent to my knowledge is that the parts you modify are protected by your copyright, but the rest remains uncopyrightable. With the exception of any chunks generated that align with someone's existing copyrighted code, as long as those chunks are substantial and unique enough.