This existing answer and comment (https://music.stackexchange.com/a/17818/23919) sort of addresses my question.
I don't know anything beyond mode I was dorian, lots of music was dorian, and Fux starts with dorian in the Gradus. But my question is whether dorian was theoretically the central or fundamental tonality? Was it specially taught that way?
Modern textbooks don't necessarily say "the major scale is the central tonality" but so many books start with a
C major scale (usually pointing out the position of the half steps) that for all practical purposes that is the modern conception.
Did teachers in the Medieval era think similarly about dorian mode? Also, did they describe dorian as 3 perfect fifths ascending and descending from a center, or two minor tetrachords, to highlight the symmetry?
In other words, was the symmetry of dorian a conscious reason for the apparent preference of the mode?