How do manufacturers rate the voltage of light bulb or any types of load?
The maximum (and minimum) voltages etc are ratings. 

A rating means that the manufacturer asserts that when operated under specified conditions, the device will give a specified performance. They back this up by testing many, many components over thousands of hours.

For instance an incandescent light might have the specified performance of >1000 hours lifetime. This will set the maximum voltage, as a hotter lamp will burn out faster. 

With a switch, the key specification will again be lifetime, as the contacts spark and erode a little every time the circuit is opened. The sparking will be worse at higher voltages, and higher contact currents. Some switches have two ratings, a heavy load and a light load rating. I saw one recently which claimed to give 10^7 operations at 100mA, and 10^5 operations at 10A.