A mod that has Variable Wattage just measurers the resistance of the atty, looks at how many watts you asked for, and uses Ohm's law to set a voltage.
With a Variable Voltage mod you set the voltage. The Universe measures the resistance of the atty and uses Ohm's law to get the wattage.
In both cases the mod outputs a voltage.
There are several different electrical circuits that can be used to alter voltage in different ways.
One is to start with a higher voltage, either use batteries in series or a simple circuit will do it, and turn it off and on quickly to simulate a voltage. Now there are 2 different ways to measure the voltage of this circuit. One is to take the average voltage over time and the other Is RMS (Root Mean Square. I am not exactly sure how this works but it can come up with a different voltage value). No matter how you measure it your coil is on and then off. It will heat up as the power is applied and then cool down as it shuts off but, even a cheap mod, will turn it off and on so quickly and the coil has a thermal mass so you will not notice it in the vape.
You can also do a circuit that will take a DC input voltage and move it up to another DC voltage. This cost a bit more than the first circuit.
You can build a circuit that will take a DC input voltage and more it down to ahother DC voltage. This is a different circuit than the one that raises voltage.
Some boards have both circuits so from 3.4 - 4.2 volts input they can produce something like 2 to 6 volts output.
Bottom line is that a regulated mod will produce a voltage, the variable wattage part includes circuitry to read the resistance of the atty and calculate a voltage to give the required voltage. In both cases the mod produces a variable voltage. Then you have to look at how the mod does these variable voltages.