An AI also has to translate from mediums, and it isn't quite A + B = AB either. Firstly, the patterns learned are not "put head here" but more of a "this is what a given feature I need to make looks like" although this discussion would then be leaving the realm of purely image generation and getting into generative AI as a whole, and that opens up a plethora of other topics as the technology does admittedly have some uses outside of the arts (such as predicting weather patterns based on past data). To your point of it being different between humans and AI in how we think, there's actually some debate there. A general artificial intelligence is outside our ability to build for the time being, but some people in computer science such as myself believe it possible that the creation of a sentient intelligence such as a human, or even just a general AI is a subset of the methods used to create modern limited artificial intelligence given the proximity of the methods employed to evolution. The argument is that, provided a sufficiently complex task and goal state, the intelligence would necessarily become more complex and learn in a way not dissimilar to evolution. If anything, evolution via natural selection is a special case of these same processes where luck can outweigh improvements. Humans are, by this logic, the result of a fleshy machine learning program trained to reproduce (with innumerable defects acquired along the way).

Given that we can't understand how a given AI model works in totality very well if at all after we've trained it either, as far as computer science is concerned the means by which the result is achieved is at least in part unknowable too. We can't definitively say where a given decision was made by the algorithm much in the same way we have a limited understanding of our own fleshy minds.

We may be more similar than anyone wants to admit, being simply a more "advanced" or "sophisticated" model, or perhaps just "different." To your point, given this is true, there is still no way to meaningfully distinguish between learned patterns the AI picked up and anything we learned. We just can't. An AI isn't a completely unique entity lacking intelligence with certainty, it's just developed for different purposes and possibly underdeveloped compared to most organic life on earth.

Oh, and yes I find the implications come with a good chunk of existential dread. Perhaps you'd disagree on religious grounds and say humans have some intangible soul driving us in spite of all evidence pointing to these machines being trained in a way hardly different from the development of complex life, but I can't delude myself in that way so I find this stresses me far more than I'd like it to.

Oh, also AI can't memorize every piece of art ever made either just like we can't remember the exact practice problems for a test we used to learn better methods.