There are nuances, so if you create a macro and then that macro writes something but it is completely determined by you then it should be ok.
Following your logic you couldn't use any third party library open source or not since you don't own copyrights to them either. Can't even use an existing compiler since parts of standard library will be embedded in it's output.
I assume what's actually intended in such cases is transferring all the rights necessary so that customer can afterwards do whatever they want with software without your permission, including making modifications, hiring someone else to further maintain it or even reselling it. It can still be a valid requirement not to depend on any commercial libraries which require temporary licensing or otherwise restrict customers ability to do what they want with combined software. Same applies for open source libraries with restrictive license (especially stuff like GPL).
When no one owns copyrights - everyone does. Both you and you custom have full rights to copy and distribute those parts of software as do everyone else, you just don't own exclusive rights (copyrights) to control whether and how anyone else can also copy those parts of software. Do you own copyright for number "10", does it mean you can't use it in your software.
The potentially problematic part is when you are trying to sell a commercial product and someone "pirates" it. If it's not copyrightable there is no piracy. In practice even largely AI generated software will contain some copyrightable parts, but the enforcement will probably still get a lot messier and no legal team wants that. In theory some could only copy the non-copyrightable parts and substitute the parts which weren't AI generated.
Yes. It can be an issue depending of the wording of your agreement with the customer. For example, if 'you' agreed to develop a piece of software 'exclusively' for the customer, and then use AI to create substantial parts of the software, then neither it was 'you' who developed that, nor was it 'exclusively' for the customer as you can't grant exclusivity.
On the other hand, if ‘you’ had taken no action at all, then there would be no software at all. The actions by ‘you’ are necessary for the software to exist, so the argument must be about whether those actions count as development or not. Is the definition of development written down anywhere?
I think it is, but I'm not a German lawyer, so I'll just link what I did in another comment - it revolves around the question who is the Geistiger Schöpfer (lit. spiritual creator) https://sta.dnb.de/doc/RDA-E-W135
Definition? Yes, but it's required over a hundred years of jurisprudence to apply it to different scenarios, in the US at least. It's amusing that you think the definition would clear things up.
> There are nuances, so if you create a macro and then that macro writes something but it is completely determined by you then it should be ok.
How far does that extend? Like would IntelliSense cause your code to not be copyrightable? It's not that different from AI autocomplete on principal level. It shows you some options, but you make the final decision what to use.
And what about binaries? These days there are not many people who could tell the exact binary that is produced by certain source code.