Hey, here’s a video from a guy who makes very interesting videos about how CG was done in movies:
https://www.youtube.com/watch?v=7N5SpsD71FI
It’s in French but there are English subs.
I really like what he does, he always interviews people involved in creating the effects he’s talking about! And there’s also a video about the Special Edition trilogy on his channel (in which he might talk about the CG trench effect) :3