Or in the case of birds. They are flying around in three dimensions, and they’re very aware of how close their neighbors are, how fast the neighbors are going, which directions the neighbors are pointing. They don’t have eyes on the backs of their heads, so they’re not so aware of who’s behind them.
But, like I said, there are simple rules about what a bird will do in response to a neighboring birds based on how fast they’re going, how close they are, and which directions they’re pointing. And then if you make computer simulations of what you’d expect, each bird is following these simple rules. You get behavior that looks exactly like what real flocks look like, including if they’re flying around obstacles such as buildings or trees. You don’t need a leader.