The mylar is farthest to the inside, to reflect the heat from the fire back towards you.
A tarp or another layer should be next..
The plastic is on the outside as a windproof outside layer. the plastic is also the "window" that will allow the long wave radiation in, but keep in the heat from getting back out.

In the best of all possible worlds, you would have an air gap or some other form of insulation, but most of them tha tI have seen online do not.

Honestly a regular super shelter is nowhere near as good as a hogan if my reading is right- the hogan is made from sticks, but looks like they are sealed with mud between the branches, so the hogan has some built in insulation- whereas unless you have a good insulation layer a super shelter turns into an icebox without some heat source. The cool trick that the super shelter has is the front window that works like a greenhouse..