You might instead be thinking of "how are different participants in a
single internet exchange cross-connected to each other?" -- in which
case the answer is "through in-building wiring that often even the
building owner isn't entirely aware of what path the connections are
taking." ^_^;
actually, i am amazed by the extent of "remote peering." if one
measures rtt to all the peers on the six, for example, the curve goes
out to well over 200ms. the six has seen remote peers from the gulf
states, and i do not mean louisiana.
graph below is one way to visualize ix connectivity, the op's question.
randy