We understand the outcomes well enough. LLMs converge onto a similar process by being trained on human-made text. Is LLM reasoning a 1:1 replica of what the human brain does? No, but it does something very similar in function.

I see no reason to think that humans are anything more than "a pile of statistics that can mimic human speech patterns if you don't tax them too hard". Humans can get offended when you point it out though. It's too dismissive of their unique human gift of intelligence that a chatbot clearly doesn't have.