While part of the answer is relating to the metric of the route, it is not the only detail that dictates the path. In part of the route table, you can see where the destination network is (with a subnet mask), and which interface to talk to it on.You can specify more 'specific' routes to take, and the most specific rules will prevail. Setool box.
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For example, if you had a home network at 192.168.0.0/24, your default route table knows to use Ethernet. If you add a direct route to 192.168.0.0/25 to be routed through your Wi-Fi adapter, then any hits to 192.168.0.0 through to 192.168.0.127 will go through your Wi-Fi, and the remainder of your network through Ethernet. If you Default Gateway (next hop) is 192.168.0.1, then your default gateway will connect over Wi-Fi, as that is where the 'most specific route' exists.
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Likewise, if your gateway is 192.168.0.254, it would go through your Ethernet.OpenVPN uses this method as well. Instead of replacing the 0.0.0.0/0 route (default route), it creates two routes - 0.0.0.0/1 and 128.0.0.0/1 - that route through the VPN interface. This way, if the VPN interface has issues, it can fall back to your standard Default Gateway.