Basically where can I find the citadel from Half life 2. I am unable to find the mdl for it or a map with a proper citadel, also where can I have some more half life maps, specifcally looking one in the citadel at the end of the game with breen. Unable to find it. I see posters and animations about it but unable to find it myself.

Also is this a greenscreen or somthing else: the background:
AT END: https://www.youtube.com/watch?v=FwnTmPtSPYk&t=88s
https://www.youtube.com/watch?v=IMwi5E3qRnI