Explain airbricks to me..

[Flibr]

Our house has a large extension built by the previous owner. The other day whilst in the garden, I noticed the airbricks along the back wall are all higher than the level of the interior floor, by about 6 or 8 inches. How does that work then? I thought airbricks were to ventilate the subfloor. I'm even more because the extension is built on a large concrete slab. Also, the rear wall which these airbricks are fixed in is made of solid blockwork (no cavity) and celotex or similar is fixed on the exterior of the building & rendered over. Is there any point to my airbricks? I have the planning permission/building regs certificates so I know it passed all the inspections. I'm just confused about the construction.

[PigletJohn]

it's possible to have airbricks which have a Z-shaped duct meaning they are higher on the outside than on the inside. What are the floors made of in this extension?

Poke an unfolded coathanger into the airbricks. Does the duct go downwards? If not, how far can the wire go before it stops?

There was also, for a time, a practice of putting airbricks at high and at low level to ventilate the cavity. This means the cavity is always cold and increase heat loss. It is a practice that has now been discontinued.

[PigletJohn]

p.s.

there might be a duct going under the new extension and into the subfloor of the old house, to allow flow of air from front to back.