What would happen on using an Ethernet cable connected to one network and then plugging in another Ethernet cable but using an USB Ethernet adapter from a different network. Would it connect to both or what ?
Assuming you plug the "Ethernet cable" into an onboard NIC, the USB adapter just represents another NIC with another network connection. This makes the host multi-homed, connected to both networks.
Note that the host most likely gets a second default gateway. Which default gateway is actually used depends on the interface metrics configured manually, by the OS or by DHCP. Note however that host configurations are off-topic here.