We don't understand how human neurons in particular are so good at what they do. We have some understanding, which has been mathematically approximated with back-propagation algorithms and transformers. That's why we have diffusion nets and LLMs that are as good as they are.

But the nuts and bolts of how neural systems give rise to reasoning or even what consciousness is... we only know that neural systems are responsible for reasoning, and we don't even know definitively that neural systems cause consciousness. We only know that if you suppress their activity consciousness is retarded, and if you damage them enough consciousness is snuffed out.