The Device Connected and Port Connected values are populated based on the Connects_to (Device Connected) and Links_with (Port Connected) associations between two models.
What happens when the Interfaces tab is viewed is Spectrum reads the Connects_to and Links_with associations for each interface model and if found, displays the model name of the model that shares that association with the interface model.
This is done dynamically and is not stored in the database as an attribute value.
You can verify this via CLI with the "show attributes" and "show associations" commands. From the show attributes of an interface or device model connected to another device you will see that there are not attributes that contain that model. On the contrary, if you list the associations you will see the type of relation (Connects_to, Links_with, etc...) and the name of the other connected device.
The CLI commands are:
show attributes mh=<model_handle>
show associations mh=<model_handle>