I like to think that miraidon is actually covered in scales that are packed together in a way that makes it look metallic.
While the eyes perhaps actually have two eyes lids. The main one closes in a way that makes the actual eye look more like a screen turning off.
The “rockets” when riding are actually pouches that can access the part in the body that makes a Pokémon breath fire for when they use an attack that uses fire
Yeah, I know there are Pokémon with metallic skin already, like metang, for example. But I think scales suit miraidon more than a metallic body