A phone can report very low ISO because of how its camera is designed, not because it is more light-sensitive than a larger camera.

Smartphones usually have a fixed, fairly wide aperture (for example around f/1.8). Since the aperture can’t stop down, exposure is controlled mainly with shutter speed and sensor gain/ISO. In bright light, if the shutter is already near its limit, the camera may need a very low ISO value to avoid overexposure.

Larger cameras usually have adjustable apertures, so in bright light they can simply close the aperture instead of needing such low ISO. Their design priorities also tend to favor better high-ISO and low-light performance.

Sensor size matters too. Phone sensors have very small pixels, which collect less light per pixel, so their true low-light sensitivity is limited by physics. Bigger sensors have larger pixels with different full-well capacity and dynamic-range tradeoffs. That’s why a DSLR may not offer ultra-low ISO even though it performs much better in low light overall.

In short: low ISO numbers on a phone are mostly a consequence of fixed wide apertures and exposure management, not superior sensor sensitivity.