how do you reflect a shape over the x or y axis when its already on the line, like the preimage is already on to of the line, how do you reflect it?

Answer 1

if the original point is on the line of reflection, it just stays there.

Answer 2

but what if none of the points are on it, but the shape it makes is, like a triangle, two points on one side, another on the other.

Answer 3

just consider each point. The shape will remain unchanged, just flipped. Just cut out a triangle, and flip it over. Has anything actually changed?