Pop's desktop environment looks like a barely customized Gnome3 session to me. I'm not sure I would even call it a fork. Looks more like just a theme.
Pantheon (Elementary) is more of a fork of Gnome3, reusing some low level parts but presenting a much different experience to the user.
They have collaborated with elementary in a few areas though. Instead of gnome software, they have Pop Shop, which I believe is a fork of the elementary app center.
They also use their own installer, which I believe was a collaboration with elementary os.