The bridgeO is the Thunderbolt bridge that bridge 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.