I will vote for the best answer based on the use of simple vocabulary, I don't know much about transformers so try to keep it as understandable as possible. I know it's not a simple subject, thanks!
They have been designed by very qualified engineers, more so than I, and have stood the test of time.
Try the site below.
But the varying magnetic field will also induce currents in the metal core of the transformer. These are called "eddy currents", are a waste of energy, and will result in excess heat. By using thin laminations for the core, and insulating the laminations from each other, transformer designers can minimize these eddy currents, and keep the transformer efficient.
Don't forget my "best answer".