While in the womb, billions and billions of neurons are born inside the developing brain. When an infant is born, they already have most of the neurons they will need throughout their lives. Learning happens when connections between neurons are formed. Because infants have so much to learn, babies are born without all the connections already in place. Some connections are there, but many still need to form.
You can think of this process a bit like a growing forest. A young forest is filled with saplings, but the tiny trees haven’t grown big, arching branches yet. As the forest grows, the branches fill out, forming a dense wood. In the brain, the same thing happens. There are billions of neurons in the infant brain, but those neurons haven’t formed all their connections yet. As a child grows and learns, a dense ecosystem of connections forms in the brain.