A Venturi is connected to the inlet of the blower. All combustion air flows through the Venturi. The outlet of the 24 VAC gas valve connects directly to the side of the Venturi. Fuel gas flows from the outlet of the gas valve into the side of the Venturi directly.
Inside the Venturi there is a cone shaped restrictor that constricts the air passage to the blower inlet. As air enters the constriction point it’s velocity increases. A pressure drop occurs at this point and creates a negative pressure in the cavity between the cone shaped restrictor and the Venturi housing. This negative pressure “pulls” gas from the outlet of the gas valve into the blower where it is mixed with combustion air and then supplied to the burner. See the images below.