Hey - i was wondering where to find or how they did the Entrance to Fort Joy in the Main Campaing, or the Exit from the Castle in Fort Joy. Those Arcs are nowhere to be found in the Recource Manager. I also tried to load up the Fort Joy Map in the Editor and select those items but you cant select the Arcs. Does anybody know why this is?

That's a wall construction, specifically '2M_Humans_Prison_A_5H_Border' with another wall construction up top for the rubble, as demonstrated in this photo.

Nope thats not the one, i mean the archway itself - i cant find it. The 2M_Humans_Prison_A_5H_Border you mentioned are the walls itself. But not the Archway.

They are both. You can select an individual wall part and scroll the mousewheel to swap between wall-types. One of them is the top arch. The lower half is also just a matching different wall variation and they're stacked on top of each other.