High ISO itself is not the main reason the signal-to-noise ratio gets worse. The bigger issue at night is that there is much less light, so the sensor records less signal to begin with. With a weak signal, noise becomes more visible.

Raising ISO on a digital camera mostly means amplifying the sensor’s signal before digitizing it. That amplification also boosts the noise already present, so the image looks noisier. In some cases the electronics and longer exposures can add extra noise as well.

So the usual chain is:

• low light at night = less signal
• less signal = poorer signal-to-noise ratio
• higher ISO = more amplification of both signal and noise

High ISO is still useful because it can boost the sensor output enough to avoid additional problems later in the imaging pipeline.

Camera design matters too. Larger and newer sensors generally perform better in low light, often because they gather light more effectively and handle noise better than smaller or older sensors.