The bridge0 is the Thunderbolt bridge that bridges together the various Thunderbolt 3 adapters in your Mac (in my MacBook Pro 2018, there's 4 of them, all members of bridge0).
Ethernet is a buit-in feature of thunderbolt 3.
To bring this bridge active, you need to bring a Thunderbolt Ethernet interface up, so connect a device that use this feature.
Note: the standard Apple Ethernet adapter is really a USB-C to Ethernet adapter, not Thunderbolt, and doesn't bring up the Thunderbolt interface (it creates a separate interface).
If you really want to bring the bridge up (why?), you can probably add another interface to the bridge (like the USB-C to Ethernet adapter) and connect this interface. You can give it a shot using the brctl command.
