Babies are born with the capacity to learn language, motor skills, etc. The brain is very structured at birth for these capacities. So no, it isn't random, but probabilistic. Meaning the structure of the brain and connections are probabilistically formed via evolution. Or is this what you meant by semi in semi-random?

In ML, think of evolution as hyper-parameter tuning. Most ML models have a defined structure but are tuned via learning. Pruning is probably such a tuning strategy in the brain.