Some AVR chips have an internal amplifier. I suggest you lookup the datasheets of the microprocessor used on the different Arduino models. That should have the answer. Precision is how much it is actually deviating from the real thing. With ADC it is usually measured in how many LSBs the slope, the curve or offset is off. Resolution is in how many small steps (fine grained) you can measure it (most significant bit count). It does not say anything about how close to true your measurement is. Don’t fool yourself in thinking the last one is all important. In the end precision depends on the voltage reference as well.