>Where do you think these “brain waves” come from?

Propagation delay and circular loops. No it doesn't have to be "external" stimuli. There's no explicit input and output layer in how a real brain is constructed. That is an artificial constraint put on artificial neural nets to make a programmers job easier. A real brain can and likely does have self stimulating loops.

>Just as transistors are fundamental to processors, neurons are fundamental to brains

Transistors are also quite simple as individual units and the computational complexity comes about in how they are connected.