How do ISPs route traffic geographically within their AS?

Sorry if this question is to basic for this sub. I was looking at AS6939 route servers and noticed that on the westcoast (Fremont) the best route to 1.1.1.0/24 would have a MED 15 but that same route in Toronto would have MED of 614. How do ISPs go about changing the MED or local-pref of routes on only certain routers in their AS? I guess when that route in Fremont gets reflected to Toronto the router knows that route is far way and changed the MED? How do they do this? Communities?