It us not. I programmed Accu controller. The best way is to track the enery coming in and out. Using only voltage is imprecise bevause the voltage/charge graph look a bit logarihmic, so the more charge you have, the more presision you need to have while measuring charge.This graph as well as total possible energy capacity and ability to Charge depend on Temperature. Also you normally have multiple cells that age differently.

I think it varies with the age of the battery and the operating temperature (probably other things too). Although if they were clever they could probably incorporate those parameters as well..

They use coulomb counting, along with an estimate of how much safely-usable energy is left in the battery.

That estimate is usually accurate, but it can drift a bit over time if you never fully recharge or discharge the battery. Also, when the battery gets old it starts having trouble delivering full current when mostly discharged, which is why you'll sometimes see shutdowns at 5-15% remaining.