The box braids are generally three-strand plaits where a particular section of the hair is divided into three parts and intertwined. The difference between a typical braid and a box braid is that hair is usually added and tight tension is applied throughout the braid creating a very polished final look with no loose strands.
If you think of the similar thing happening with the cornrow braids, the difference just being instead of the braid going along the scalp, only the root of the braid is to the scalp in a square or similar shaped section.
Yes, all the hair is divided into proper squares, and each hair box/square is plaited down through the length to give you an individual solid braid out of a box in a uniform thickness.
This way, you get equal proportionate braids throughout your head that look similar, giving you complete box braids.
As your hair is divided into uniform squares, each having a block, more length and volume are required to finish the braid. At that time, the hair bundles or extensions are used in braiding to provide thickness and length.