The seitzer style dispenser is the way to dispense the soda to the wine that is already in a normal tumbler or high ball glass. The wine is usually a dry white, e.g. rizling, not top-of-the-line. We have a saying “it's acceptable with soda” for “good enough”, comes from the fröccs.

The purpose of adding soda is to make it more refreshing in the Summer and allows to drink more without flipping over.

About the ratios: https://hu.wikipedia.org/wiki/Fr%C3%B6ccs is a good list. bor = wine, szódavíz = carbonated water.