See here  for the main micropython documentation.
All ADC readings are referenced to the supply rail, which is about 3.3V. To increase the ADC accuracy, the internal voltage reference can be read, and used to offset variations in the supply rail.
adcin = pyb.ADC(channel_to_read).read() ref_reading = pyb.ADC(0).read() supply_voltage = 4095/ref_reading*1.21 # or change 1.21 with the calibrated value - se below adc_voltage = adcin / 4095 * supply_voltage
Note, the factory reads the internal reference with a supply voltage of 3.0V, and stores this reading. This can then be used to get the actual voltage reference. For example
import stm factory_reading = stm.mem16[0x1FFF75AA] reference_voltage = factory_reading/4095*3