Hi Nibal,
We usually go in-grade under mulch (rock or wood. In some areas, people place below grade when there is a threat of rodents chewing the tubing.
Our supply and exhaust headers are made of PVC and are placed 6" below finished grade. Risers go to the surface where we use fpt x barbed fittings to connect to the poly tubing. We try to keep our areas served by the pipe transition locations (which we show on our plans) to not more than 7gpm so that we can use 3/4" supply headers. This keeps the areas more manageable. Also, this allows us to use pressure regulators between zones served by a single valve. I hope that this helped.