Every analogRead() call in an Arduino sketch leans on a piece of hardware doing real work behind the scenes: an analog-to-digital converter, translating…
An explanation of the ESP8266's ADC characteristics: 10-bit resolution, a single pin, and the voltage range difference between NodeMCU and the bare chip.
Two ways to measure voltage with Arduino: direct measurement up to 5V, and a voltage divider for higher voltages, with the full calculation formulas.