A divider made of water

Two resistors, one of them water

The whole board is one circuit. VCC goes to the VCC loops. Water, when there is any, joins them to the SENSE loops. From SENSE, the 1 MΩ goes to GND, and the 150 Ω goes on to SIGNAL.

That is a voltage divider with the water as its top half:

SIGNAL = VCC × 1 MΩ / (1 MΩ + water)

Slide the water from a trace of nearly pure condensate on the right to a salty puddle on the left. Around 1 MΩ, where a fogged board sits, the reading moves most for a change in the water. Far to the right it barely leaves 0; far to the left it is stuck near VCC.

The 150 Ω is in the path too, but next to a megohm it changes the reading by less than a count, and at rest no current flows through it at all. Only the water and the 1 MΩ decide SIGNAL.

VCC is your logic voltage

With the water near 0 Ω, SIGNAL is VCC. So VCC must be the voltage your board's analog pin takes: 3V3 on an ESP32, an ESP32-S3 or a Pico, 5V on an Uno. The page this replaced offered either supply as though it suited any board; 5V on an ESP32 can put 5 V on a 3.3 V pin.

On an Uno and a Pico there is a bonus. Their ADCs measure against the same supply VCC comes from, so the count is the divider's share of full scale: 1023 × 1 MΩ / (1 MΩ + water), whatever the USB supply is doing. An ESP32 measures against a reference of its own, so the count moves a little with the 3.3 V rail.