Real neurons are orders of magnitude more complex than their artificial pseudo-approximation (it is all based on the century-old understanding of how neurons work). You can think of _individual_ biological neuron as an analog of the small artificial neural network. You can see this simple visual explanation on YouTube[1]. So we aren't even close. It doesn't mean the AI is impossible, it just means people underestimate the "computing power" of real brains, as well as that AI, even the future one might be totally different in how it works from the natural intelligence.

[1] https://www.youtube.com/watch?v=hmtQPrH-gC4

But for biological neurons to do something that can't be efficiently approximated on a digital computer (but conductive to useful information processing) they need to have unknown unknowns (well, partial unknowns like an unknown quantum algorithm will do too).

We don't know the violations of the physical Church-Turing thesis that are conductive for machine learning. We don't have evidence for their existence in the brain (although, the brain would be the prime candidate for finding them as evolution works directly with the true physical laws).

BTW, large ANNs don't try to model how the brain does things. They are trying to mimic what the brain does. So, using "how many transistors/artificial neurons it takes to model a biological neuron" is not a good approach.