Nominal battery voltages depend on the chemistry of the cell used, specifically the “oxidation potential” of the cathode and anode materials. You can think of it as “how badly one side of the battery wants to grab electrons from the other side”. 3.7 V is extremely common as it’s the number for common lithium materials. (But the real voltage varies a bit depending on the charge). Multiples of 1.5 V are common because it is the number for alkaline cells and you can get multiples of the voltage by putting several cells in series (end on end). I dont think there are commonly used chemistries with a nominal voltage of 2 V.
There’s no such thing