Concrete stairs substrate preparation and layering
If you are building the stairs tight to the house, you need to go to the frost line with your concrete foundation. Run cinder block to just below the grade level, then go to the brick finish. If the stairs are small enough, just pour the concrete just below the grade and start your brick right from there. Many stairs have been built with a dirt infill then 4" of gravel over that, all on top of the footing slab. I feel that is an error in doing it that way since the dirt can take on moisture and freeze in winter. All gravel under the pour for the stairs, on top of the footing slab will keep heaving and cracking from freezing temps.

The plastic under the concrete also help slow the rate the water escapes the concrete, in essence keeping the concrete better hydrated, making it stronger. Foam panels will do nothing for it.