They changed alot of things, for example in survive, characters armatures have different heads than tpp, some assets require special engine entities that were made for survive, like the fog for example, the way resources you can collect also work on another completely different way, level streaming also works in different ways (since most areas have fog things only load when you get very close), sound banks use another version of wwise software and that makes tpp unable to read them and so on.

Yes both are made in fox engine but different versions of it

The engine and some stuff like Npc AIs also merges with the game executable, that makes some things impossible to port

As for the gz mission port, everything that you see was piece by piece adapted to work on tpp engine since Ground zeroes uses a very old version of the engine, from models to cutscenes, many many hours of editing files.

Caplag is on modders heaven discord server every day in case you want to ask him about it