I just recently picked this book up. I heard of it before from reading fictional book called One Second After by William R. Forstchen. Although it is fiction, the author is known to be fairly realistic and brings up the concerns we have with the grid.
I am not an Electrical Engineer, but I do have a degree in Electronics Engineering Technology. We deal with smaller components and DC,not AC. But a small transformer is a coil of wire around either an iron core or other material on one side (primary) and then another coil of wire around the other end of the iron core/material (secondary). The number of windings on each coil determines if the voltage is stepped up, down, or the same. So, I would imagine the bigger ones are an extremely giant form of this, but using special material to make them more efficient (if that is possible). This doesn’t break it down precisely, but maybe helps to understand the concept behind the transformer. I would like to think that in modern times, they could replace them with much smaller transformer. The other reason is for the raw material, such as copper and special steel to make the transformers.