Most windows, because of cost, are made from cast concrete. A form is made and then repeated over and over. The size of the form is about 1'x1'x1' which will make nice blocks. And because they are concrete they can easily be stacked and support load.
I am not sure why villagers opt for concrete blocks over pre-fabricated windows (which are much different than the standard pre-fabs in the USA or anywhere else I can think of). It is most likely cost.
In whichever case what I have observed is that villagers have taken it upon themselves to create custom forms on which to pour the concrete. What has come from this is different patterns expressed in different villages. One of the unique aspects of the people expressed through the architecture.
Here are a few images of these windows:
