A Drosophila melanogaster has about 135k neurons, and we only managed to reconstruct its connectome in 2023.
The human brain has 86 billion neurons, about 1 million times more. Therefore, it is obvious that we are very very far away from a full connectome.
Instead however, we could look at larger scales of connectome, and then try from that to extract modules, and then reverse engineer things module by module.
This is likely how we are going to "understand how the human brain works".
Some notable conectomes:
This is the most plausible way of obtaining a full connectome looking from 2020 forward. Then you'd observe the slices with an electron microscope + appropriate Staining. Superintelligence by Nick Bostrom (2014) really opened Ciro Santilli's eyes to this possibility.
Once this is done for a human, it will be one of the greatest milestone of humanities, coparable perhaps to the Human Genome Project. BUt of course, privacy issues are incrediby pressing in this case, even more than in the human genome project, as we would essentially be able to read the brain of the person after their death.
As of 2022, the Drosophila connectome had been almost fully extracted.
This is also a possible path towards post-mortem brain reading.
Figure 1. Source. Unconfirmed, but looks like the type of frozen brain where a Microtome would be used.