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I believe, by default, we have the ADC voltage references set to 3.3V and GND (internally). This frees up as many analog inputs as possible for users to use.
Yes, by default the analog references are set to VCC and GND internally. The Vref+ and Vref- pins of the ADC are sometimes available on a board's headers, and sometimes not. For boards with an Arduino-like footprint there is a dedicated AREF pin which is connected to the Vref+ pin. This is never connected to anything else. The Vref- pin may be available or it may not - check the manual for your chosen board, since they are all different.