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On the ATmega328P (the microcontroller of the Arduino Uno), the AREF pin is initially floating. If you try to read its voltage, you can get basically anything.

As soon as you select an analog reference other than EXTERNAL, the AREF pin gets internally connected to either the AVCC pin (if you select DEFAULT) or the internal 1.1V reference (if you select INTERNAL). This typically happens when you call analogRead() or, in your example code, when you set the REFS0 bit of ADMUX.

Edit 1: As pointed out by JRobert in a comment, the section ADC Voltage Reference of the ATmega328P datasheet is worth reading if you consider using the AREF pin. Especially the warning in the third paragraph.

Edit 2: Answering this new point:

AREF can only output a voltage when used by the ADC, but not when used by the AC.

Whether or not it is being used by the ADC and/or the AC is irrelevant to the behavior of the AREF pin. The only thing that matters is whether or not the REFS0 bit of ADMUX is set. The pin outputs a voltage if and only if that bit is set.

Note that you can change this bit without ever really using the ADC, although that would probably be useless.

On the ATmega328P (the microcontroller of the Arduino Uno), the AREF pin is initially floating. If you try to read its voltage, you can get basically anything.

As soon as you select an analog reference other than EXTERNAL, the AREF pin gets internally connected to either the AVCC pin (if you select DEFAULT) or the internal 1.1V reference (if you select INTERNAL). This typically happens when you call analogRead() or, in your example code, when you set the REFS0 bit of ADMUX.

Edit 1: As pointed out by JRobert in a comment, the section ADC Voltage Reference of the ATmega328P datasheet is worth reading if you consider using the AREF pin. Especially the warning in the third paragraph.

Edit 2: Answering this new point:

AREF can only output a voltage when used by the ADC, but not when used by the AC.

Whether or not it is being used by the ADC and/or the AC is irrelevant to the behavior of the AREF pin. The only thing that matters is the value of the REFS0 bit of ADMUX. The pin outputs a voltage if and only if that bit is set. This can be clearly seen in this part of the schematic of the ADC, from the datasheet:

When REFS0 is 1, the transistor circled in red connects the AREF pin to either AVCC or the internal reference, as selected by the multiplexer controlled by the REFS1 bit.

Note that you can change this bit without ever really using the ADC, although that would probably be useless.

On the ATmega328P (the microcontroller of the Arduino Uno), the AREF pin is initially floating. If you try to read its voltage, you can get basically anything.

As soon as you select an analog reference other than EXTERNAL, the AREF pin gets internally connected to either the AVCC pin (if you select DEFAULT) or the internal 1.1V reference (if you select INTERNAL). This typically happens when you call analogRead() or, in your example code, when you set the REFS0 bit of ADMUX.

Edit: As pointed out by JRobert in a comment, the section ADC Voltage Reference of the ATmega328P datasheet is worth reading if you consider using the AREF pin. Especially the warning in the third paragraph.

On the ATmega328P (the microcontroller of the Arduino Uno), the AREF pin is initially floating. If you try to read its voltage, you can get basically anything.

As soon as you select an analog reference other than EXTERNAL, the AREF pin gets internally connected to either the AVCC pin (if you select DEFAULT) or the internal 1.1V reference (if you select INTERNAL). This typically happens when you call analogRead() or, in your example code, when you set the REFS0 bit of ADMUX.

Edit 1: As pointed out by JRobert in a comment, the section ADC Voltage Reference of the ATmega328P datasheet is worth reading if you consider using the AREF pin. Especially the warning in the third paragraph.

Edit 2: Answering this new point:

AREF can only output a voltage when used by the ADC, but not when used by the AC.

Whether or not it is being used by the ADC and/or the AC is irrelevant to the behavior of the AREF pin. The only thing that matters is whether or not the REFS0 bit of ADMUX is set. The pin outputs a voltage if and only if that bit is set.

Note that you can change this bit without ever really using the ADC, although that would probably be useless.

On the ATmega328P (the microcontroller of the Arduino Uno), the AREF pin is initially floating. If you try to read its voltage, you can get basically anything.

As soon as you select an analog reference other than EXTERNAL, the AREF pin gets internally connected to either the AVCC pin (if you select DEFAULT) or the internal 1.1V reference (if you select INTERNAL). This typically happens when you call analogRead() or, in your example code, when you set the REFS0 bit of ADMUX.

On the ATmega328P (the microcontroller of the Arduino Uno), the AREF pin is initially floating. If you try to read its voltage, you can get basically anything.

As soon as you select an analog reference other than EXTERNAL, the AREF pin gets internally connected to either the AVCC pin (if you select DEFAULT) or the internal 1.1V reference (if you select INTERNAL). This typically happens when you call analogRead() or, in your example code, when you set the REFS0 bit of ADMUX.

Edit: As pointed out by JRobert in a comment, the section ADC Voltage Reference of the ATmega328P datasheet is worth reading if you consider using the AREF pin. Especially the warning in the third paragraph.