Interpreting this ADC circuit
But mostly I don't quite understand what they are saying about switching resisters.


that's because of the dynamic range of the sensor, from 1k to 1M -> if you were to pick a fixed resistor, you would be out of range at either the high end or the low end, on an 8-bit adc.

you may not need that for a 16-bit adc. do your math to be sure.