customizing 3D game kit

I have a game that’s pretty good and pretty far along and would like to add game stuff like menus and game play such as outdoor experiences like moving environment grasses and rocks, snakes, cliffs, moving grass, dialog and other specific typical gaming stuff. I saw that the unity 3D environment kit has a lot of stuff that would be valuable to add. I looked into it and am trying to adapt the stuff that is in it to my game, but I am having trouble figuring out how to customize it. What is the best way to go about customizing the untiy 3d game kit. I searched all the documentation that I could find about it but it seemed to only show how to use it and not how to add your own meshes, animations, etc. Is there more that I am missing?

You might want to start here with this free no-code game development course for beginners. It’s a free tutorial series that walks you through how to use Unity’s 3D Game Kit. I think it covers most of the issues / modifications you mentioned above.