A brain is a biological organism built essentially by itself from the instinct-like instructions within the parts that become to grow to become it. This biological process is ongoing never-stopping from the very first neuron activation to the very final last. The human brain literally evolves itself out of a basic stupor to a high-functioning instinct-overcoming decision machine and then devolves right back down to a senile partial-function torpidity before finally shutting down.
A brain is also a mostly subliminal memory-experience collecting/tabulating/cross-referencing/altering/deleting/hiding/inspecting/searching/decision-making-using collection of co-editoring neural signals.
Your brain is also the only organism to give itself a name. Or rather, it was that wildly surging mob of neurons which did it.
You are a far larger thing than the sum of your parts. If you ever feel overcome by the enormous complexity of life outside yourself, just remember how much actually has to happen inside in order for you to make a simple cup of hot coffee.
And remember to let that hivemind within enjoy that coffee.