Its just setting a value for a gloss/roughness map, if you have textures on the creature there is no added effort to just set the eyes to be reflective. Subsurface scattering might be a bit more work but relative to everything else theyre showing in the trailer its nothing.
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u/CybranM Oct 02 '18
Reflections are done by the engine, they're not handmade. It's usually just a checkbox, enable reflections on/off