Transformer works on the electromagnetic induction concept. So we need a strongly established flux flowing through CORE of transformer. To make this task easy we have to choose a material which has no reluctance or less reluctance to magnetic flux. read more
Also transformer works on AC , so flux is produced by AC is alternative in nature hence we need a ferromagnetic material which allows the reversal of this flux produced by AC. since Platinum is paramagnetic material and steel or iron is ferromagnetic materail, we use steel/iron instead of platinum. read more
As far as I know, every transformer in the movie is computer generated. There are no animatronic robots if that's what you mean. read more