Atavism , is modular. So there is no problem to use all your own, even on the contrary it is supported. There are a lot of examples inside and you shouldn't treat them like a finished game, all that is provided by the developers is a demo.
Developers also have many of their own Unity assets to work with.
With Unity 6, I can only say one thing, be careful with your scripts because they can ruin your project.
The answer was above from Martin - you can use absolutely everything you have. I'll just add that you can also completely make Atavism for yourself, because there is practically no source code for everything.