Why is an unconnected interface Up

The loopback internal command may be configured on the interface to enable internal loopback detection. Therefore, the interface is Up.

You can run the display interface gigabitethernet 0/0/1 command in any view to check whether internal loopback detection is enabled on GE0/0/1.
display interface gigabitethernet 0/0/1
...
Port Mode: FORCE COPPER
Speed : 1000, Loopback: INTERNAL
Duplex: FULL, Negotiation: ENABLE
Mdi : AUTO, Flow-control: DISABLE
...The command output shows that internal loopback detection is enabled on the interface.

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