There are three basic areas we discuss for Arduinos. The Arduino microcontroller, The circuit, and the software interface to program the microcontroller Below are links to tutorials explaining each
What is an Arduino https://learn.sparkfun.com/tutorials/what-is-an-arduino/all
Basic Electronics https://www.makerspaces.com/basic-electronics/
Language Reference https://www.arduino.cc/reference/en/
Arduino homepage https://www.arduino.cc/