When both wired and wireless network connections are active, which one is used?

I imagine that most applications don't even notice that I have two network cards. They ask the kernel to open a socket and they're happy like that. Transparency is great.

I'm left wondering: when I am connected to both a wired and wireless network, which one is used by the system? The first one that was created? The last? Both equally? Both randomly? Both, with some intelligence to balance the load and find the open ports?

Related: what does the network history part of the System monitor actually displays? The sum of the debit of the two network cards?