The Connectome is the name given to the map of neural connections within an organism's nervous system. Analogous to the 'wiring diagram' of a machine. See: Wikipedia
When organisms are growing - in the sense of 'wiring up' their nervous system - it's assumed that genetic mechanisms must somehow be directing the process. Guiding which neurones connect to which others.
The details of this guiding process are unknown.
So far, only one organism - the roundworm Caenorhabditis elegans - has had its connectome fully mapped. However :
[…] the genetic mechanisms that determine which neurons can synapse with each other remain largely unknown even in this simple and well-studied organism "
Source :Uncovering the genetic blueprint of the C. elegans nervous system in PNAS, December 29, 2020 117 (52) 33570-33577