The designer determines the voltage. Turns Ratio = Voltage ratio. For example, if you want a 1:1 transformer, you, the designer, determine the frequency and voltage range. Say you want a 120 VAC @ 60 Hz 1 kVA transformer. You design the transformer to operate in the 60 Hz range, which determines the number of turns and material required in the primary and secondary. You select the wire size based on the current and insulation resistance based on the highest possible voltage for safety.
Input 120 Volts, you will have 120 volts. Input 60 volts and you get 60 volts out. As long as you stay with the design parameters, mostly for safety and voltage you want.
Use 2:1 input 240, you get 120 out. With 1:2 120 in, 240 out.
