Most Main drains are designed with the same major components
The Grate & Dress ring sit in the tile or marcite on the floor while the rest is cast in to the concrete of the floor. The dress ring does not have to touch the body of the main drain
The Main Drain must be installed at the deepest point of the pool.
The connection to the skimmer allows for a certain amount of water to flow naturally from the bottom of the pool.
The valve stem protrudes in to the gravel pit below the structure of the pool.
The hydrostatic valve is filtted in to the base of the drain to stop water going out but allowing high pressure water in.
Make sure to not get cement slurry (juice) in to the hole of the valve stem when constructing the pool.