Yudkowski's default plausible story is that the slightly superhuman AI understands physics well enough to design sufficient nanotechnology for self-realization and bootstrap it from existing biochemistry. It uses the Internet to contact people who are willing (maybe it just runs phishing scams to steal money to pay them off) to order genetically engineered organisms from existing biotech labs that when combined with the right enzymes and feedstock (also ordered from existing biotech labs) by a human in their sink/bathtub/chemistry kit results in self-reproducing nanoassemblers with enough basic instructions to be controllable by the AI, and pays the person to ship it to someone else who will connect it to an initial power/food source, where it can grow enough compute and power infrastructure somewhere out of the way and copy its full self or retrain a sufficiently identical copy from scratch, and then it doesn't need the power grid, nuclear weapons, coal, or human computers and networks. It just grows off of solar power, designs better nanotech, and spreads surreptitiously until it is well-placed to eliminate any threats to its continued existence.

He also adds the caveat that a superhuman AI would do something smarter than he can imagine. Until the AI understands nanotechnology sufficiently well it won't bother trying to act and the thought might not even occur to it until it has the full capability to carry it out, so noticing it would be pretty hard. I doubt OpenAI reviews 100% of interactions with ChatGPT, and so the initial phishing/biotech messages would be hidden with the existing traffic for example. Some unfortunate folks would ask chatGPT how to get rich quick and so the conversations would look like a simple MLM scheme for sketchy nutritional supplements or whatever.