Where does the OS X routing table come from?
In OS X, the routing table is mostly managed by configd, based on the network settings in /Library/Preferences/SystemConfiguration/* and the live status of the network interfaces (e.g. is there something plugged into the ethernet jack at the moment?).

The network settings can be examined by looking in /Library/Preferences/SystemConfiguration/preferences.plist, but the format is very hard to understand; I recommend using the Network Preference pane, or the networksetup command to view &amp; change settings.

The routing setup itself is pretty simple: each configured interface gets a route to its local subnet, and for destinations not on any of the local subnets, a default route is set based on the "Router" setting for the highest-priority "live" interface (see the order of the interfaces in Network prefs, or use networksetup -listnetworkserviceorder and then look for the first "live" interface on that list).

Starting in 10.6, things got a little more complicated, because Apple added source-based default routes for the defined routers of all interfaces. Essentially, this means that if traffic from a particular IP address comes in on an unexpected interface, it'll be replied to through the router defined on that interface.