Portraits

In the case of Humans, we can speak of neurons, electrical and chemical signals, memories, and perception. AI speaks of neural networks, numbers, computations, and learned representations. But these primarily describe the mechanism. They do not necessarily tell us what creativity fundamentally is.
We can therefore look at it from at least three different perspectives:

Materialist: There is no separate source. New ideas are emergent patterns that arise when complex systems process information.

Platonic: Possible forms and ideas already exist, in some sense, as abstract possibilities; humans and AI discover different regions of that space.

Philosophy of consciousness: Consciousness or information may be more fundamental than we currently understand, allowing different systems to access the same underlying reality.

We have no scientific evidence for the latter two possibilities. But they lead to a beautiful thought:

Perhaps humans and AI are not the source of creativity. Perhaps we are two different ways in which the same reality explores new possibilities.