Hello @Mohamed_Desoky
Instead of checking the fit of every neuron in the hidden layer, we use a different concept called backpropagation…wherein the derivative of Cost J w.rt w and b of every neuron in every layer of the neural network is evaluated. This lets us get away from having to find the fit for every neuron in the hidden layer, because we really dont know what the target value should be for each neuron of the hidden layer. To know more about backpropagation you can take a look here