Nyquist sampling in image acquisition
The digital image acquisition sensor (CMOS, CCD etc) is typically defined with a physical area of H x W (usually expressed in diagonal length in mm) and number of photo-sensitive elements; the pixels. PPI is not a part of sensor specifications for cameras, (but it can be for scanners yet you're dealing with smart phone cameras).

The Nyquist rate of an image sensor is determined by the lens resolution limit. The optical image that's formed on the sensor surface shall not achieve a resolution higher than the pixel spacing can resolve. Otherwise, a distortion known as aliasing will happen.

You don't need to know the PPI value of a senor to determine its Nyquist rate.

Note that with different digital image sizes you will always have unaliased images; this shall be guaranteed by the camera design and not the user.

However, you can apparently compute the PPI value of the sensor by dividing its number of horizontal pixels to its horizontal sensor length which is mostly a useless metric by itself.