How do crosshairs work if the scope is on top of the firearm?

The scope or "Optic" although placed above the barrel of a firearm is designed that the crosshairs or red dot intersect with the barrel at a designated range. think of the ">" symbol as a sort of reference. In a nutshell the sight will align with the barrel at a given point. If your scope is sighted at 300m and your target is closer or further than that range, you will need to adjust your point of aim to hit your target accurately.