Responsibility and ownership always lies with the humans. Even supposedly 100% AI generated content is still coming from a process started and maintained by humans. Currently also prompted by a human.
The humans running those processes can attempt to deny ownership or responsibility if they so choose but whenever it matters such as in law or any other arena dealing with liability or ownership rights, the humans will be made to own the responsibility.
Same as for self-driving cars. We can debate about who the operator is and to what extent the manufacturers, the occupants, or the owners are responsible for whether the car causes harm but we'll never try to punish the car while calling all humans involved blameless. The point of holding people responsible for outcomes and actions is to drive meaningful change in human behaviors in order to reduce harms and encourage human flourishing.
In terms of ownership and intellectual property, again the point of even having rules is to manage interactions between humans so we can behave civilly towards each other. There can be no meaningful category of content produced "100%" by AI unless AI become persons under the law or as considered by most humans.
If an AI system can ever truly produce content on its own volition, without any human action taken to make that specific thing happen, then that system would be a rational actor on par with other persons and we'll probably begin the debate over whether AI systems should be treated as people in society and under the law. That may even be a new category distinct from human persons such as it is with the concept of corporate persons.