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, then 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