How exactly is the location of contact determined on an analog resistive touchscreen?
"What determines the voltage measured (and thus the resistance calculated from voltage and current)?"


The voltage mesaured is determined by the location of the touch. No resistance is calculated explicitly.


  "What are the conducting layers lined perpendicularly?"


To make sure that they touch only at the location where the user touches the screen.


  If the whole resistive touchscreen is "analog resistive", 


that is correct


  why then "this analog voltage is processed by analog-to digital converters (ADC) to create a digital signal that the device's controller can use as an input signal from the user"? 


Because the device software can handle only digital values.


  (ALSO, ISN'T "DIGITAL" THE OPPOSITE OF ANALOG?)


That depends on what you mean by OPPOSITE. 

What you don't seem to have picked up is how a tochscreen actually works. If it is your assignment to do that, you will have to do some more study, especially the concept of "voltage divider". A basic understanding of Ohm's law as used for series and parallel circuits will also be handy.