Thunderbolt to USB 3 adapter
That makes sense. The USB adapter has a driver and is active.

Thunderbolt isn't getting a network address since you don't have a direct network on that port. You would have an active connection if you went thunderbolt to thunderbolt on another Mac or PC or a direct thunderbolt to gigabit ethernet adapter.

Don't over think the network pane - as long as the end device is listed, you are good to go without worrying about what's between it and the CPU.