The four pins

Count from the square pad

The four holes look the same. One pad is square, and it is GND. From the front, parts facing you and the header at the bottom, the silkscreen reads GND, VCC, NC and SIGNAL from left to right. The back prints no names, and there the square pad is on the right.

Pick each pin in turn. What each one reaches comes from the board's net list, not the silkscreen:

• GND reaches the 10 kΩ load and the LED's cathode.
• VCC reaches the phototransistor's collector and nothing else, so in the dark the block takes almost nothing from it.
• NC reaches nothing. Wire it or leave it; it makes no difference.
• SIGNAL is the phototransistor's emitter, with the 10 kΩ and the LED's 1 kΩ to GND.

VCC is your logic voltage

In strong infrared SIGNAL climbs to about 0.4 V under VCC. So VCC decides the most your pin will ever see: put it on 3V3 beside an ESP32, an ESP32-S3 or a Pico, and on 5V beside an Uno. 5V beside a 3.3 V board reads fine in a dim room, and can put up to about 4.6 V on the pin once sunlight reaches the sensor.

SIGNAL is analog

SIGNAL does not switch between two levels. It moves smoothly with the light, so wire it to an analog input: A0 on an Uno, GPIO 34 on an ESP32, GPIO 4 on an ESP32-S3, GP26 on a Pico. The two ESP32 pins are on ADC1, the converter that keeps working while Wi-Fi is on. These are the pins the Ambient Light Sensor book uses, so both blocks wire the same way.