How does the different network interfaces work?

Home Assistant lets you choose different network interfaces on the system settings.

I have 2 questions:

eth0 is always marked as default, but it is currently a 4G router and I wanted to use wlan0 as my dedicated default interface, but there is no way to choose the default one.

When I add more interfaces, which one is used by the NabuCasa URL? If one interface connection drops but the interface keep existing, will the other be used? If so, will it work through NabuCasa URL instantly?