@aaronpk Short answer: It is the direction perpendicular to the mirror which get flipped. Long answer:
Why do mirrors flip horizontally (but not vertically)? - https://www.youtube.com/watch?v=vBpxhfBlVLU&t=0
@aaronpk Easiest way I can think of it is that it's not flipping at all, but rather turning inside out.
@aaronpk Because our eyes are horizontal most of the time? I remember Martin Gardner had a great column about this several decades ago, but I can’t remember the details.
@aaronpk how I try to: they don't flip either, but your mind "rotates the camera" for you along the horizontal plane because that's what it understands.