It’s perfectly possible on Linux to have several network adapters with the same IP address, or several default routes.
Most server applications will listen to 0.0.0.0 address, which means all network interfaces. Any incoming TCP connection will remember it’s network interface, and the server will send responses to the same interface.
This will not work for UDP connections, and for outgoing TCP connections - they will always choose the network interface with the lowest metric, which you can print with ip r command.
This does not include advanced techniques like bridge or bonding or iptables routing - you need to run special commands in the terminal, which you cannot do just by clicking your mouse in system settings app.







Nope. The server receives UDP packet from WiFi and sends reply over Ethernet, which simply gets lost in your router. From the Linux side there’s no error, it sent the packet somewhere, and what happens next is your router’s problem.