If you have multiple WAN connections and both are set up correctly, the system will set up default routes for you. Those are a "catch-all" rule for everything that is sent.
If an interface does not have a WAN connection (or if the network should not be the default for other reasons), that default route won't be created (or rather, the router won't advertise that route) and only a "targeted route" is present.
In my case, the route list looks like this:
$ ip route
default via 10.2.0.1 dev enp9s0 proto dhcp metric 100
default via 192.168.178.1 dev wlp6s0 proto dhcp metric 600
10.2.0.0/24 dev enp9s0 proto kernel scope link src 10.2.0.2 metric 100
192.168.122.0/24 dev virbr0 proto kernel scope link src 192.168.122.1 linkdown
192.168.178.0/24 dev wlp6s0 proto kernel scope link src 192.168.178.210 metric 600
One of those connections (10.2.0.1) is a LAN connection, the other is wireless (192.168.178.1). If both are present, the number at the end decides which connection should be preferred, the lower number having higher importance.
As far as I know, a drop of one connection won't be unnoticable (since it usually takes a few seconds to recognize that a connection was actually dropped), but it should fail over to the other without issues (unless you are within a transfer, which not all clients/servers can handle).