We genuinely don't know that yet. People keep saying this as if we have any reason to suspect neural networks are even remotely related to how mammal brains work. The strongest link they have is that neural networks chose a very heavy handed name.

It's pretty likely that a handful or neurons, or even a singular neuron is comparable to a HUGE neural net in ability to "learn".

We know of plentiful single celled organisms that learn. It's clear we have no fucking clue what's going on, and we as computer people should stop letting AI researchers claim magic