HI @VaL
I think you have a lot of good suggestions in this thread on getting more detailed information - I was attempting to meet you where you are at and not overwhelm you with UN complexities right from the get-go - there is plenty of time for that. 🙂
| I assume this is part of a UN
Yes.
| asset group and asset type are domains?
AG is a subtype and you are correct that the "super-subtype" (a term of my own making to facilitate understanding) is represented by a domain dedicated to that end.
| (W)here is the voltage separation by asset group?
Transmission is HV, Primary is MV and Secondary is LV
| If you created a FC called electric line, what and where are the constraints imposed by the UN in terms of number of FCs?
A UN implementation requires each of the documented feature classes, nothing more, nothing less. If you did create a FC called "Electric Line" not only would it be confusing to the user, it would not be recognized as part of the Utility Network definition.
Hope that helps you along your way. As @RobertKrisher a schema model may be worth a "million words" and indeed that may be true - but I would encourage you to take baby steps and understand the foundational concepts before diving into the myriad of nuance and detail, It is easy to get overwhelmed - at first but, it's also totally learnable and as you will soon see, it has a lot more potential than the GN did/does.