So what do you think the guy in the forementioned mod did? How would I do it too or is it too complicated?
Vanilla game examples:
There are many locations like these, but here are two good examples:
- The Great Porch of Dragonsreach (*)
- The North-facing porch outside one of the entrances to the player home in Solitude.
In both places you can see a wide extent of the surrounding landscape of Skyrim. However, that has nothing to do with the actual playable landscape, and it is rather an unpopulated mock-up or duplicate that has been included with the cell where you are. Just type ~tcl on the in-game console and go over/through the railing to have a closer inspection.
(*) This is a prime example of creating an interior in an exterior cell, but one that is dedicated for the purpose, and not the normally playable one. Just imagine adding a transparent window to the scene, and there you have it.
And just as a curiosity: If you ever played Oblivion and had the Thieves Den DLC, it used a similar construct for a windowed room inside a larger interior cell. If you entered that room, you entered a duplicate interior cell, where you could peek through the windows at a low-quality, unpopulated mock-up of the original cell.