Basically, an ADC (hence an IIO device) that can be connected to a touchscreen (hence an input device), measures the position or the point where you touch with analog-to-digital conversion. The GRTS (generic resistive touchscreen) driver does the bridging between an ADC touchscreen and the input subsystem. What happens is that the ADC device registers channels for the touch position (X/Y coordinates, touch pressure), which are being read by the GRTS as a consumer for the IIO, and then the GRTS registers a touchscreen in the kernel, and reports touch data further on touch events.