2.2 Question: Why is V/vi Major?

I’ve just gotten to 2.2 and I’m a mite confused by this relationship. If the answer is explained further ahead in another chapter, just tell me to keep reading, but I also welcome any elaboration. Two things are confusing me:

A) First of all, if cadence is the process of resolving dissonance, which is created by distance from root, why would the cadential dominant of a major key be used to resolve to a minor vi? The interval pattern of A for instance is shared by F#m.; it doesn’t seem like it shares the same interval harmonics as the Am it’s returning to in the key of C.

B) Secondly, if I understand correctly, vi is i aeolian, meaning that the cadence would be v aeolian, aka iii ionian. Since ionian and aeolian share the same notes, it seems incredibly unintuitive and arbitrary to jump to an unrelated major scale to borrow its cadence when a perfectly good cadence for aeolian is already contained in ionian.

I’m figuring this all out piecemeal, so all insight is useful.