>but where do you think LLMs pick up these patterns?

>LLMs are able to capably mimic the common patterns that how to write books have suggested for the last 100 years as ways to make your writing more "impactful" and attention-grabbing. So are humans. They learned it from watching us.

Don't forget that LLMs (at least the "instruct" versions) undergo substantial post-training to align them with the authors' objectives, so they are not a 100% pure reflection of the distribution seen on the internet. For example, it's common for LLMs to respond with "You're absolutely right!" to every second message, which isn't what humans usually do. It's a result of some kind of RLHF: human labelers liked to hear that they're right, so they preferred answers containing such phrases, and those responses became amplified. People recognize LLM-generated writing because LLMs' pattern distribution is different from the actual pattern distribution found in articles written by humans.