You need to select all of the props in the scene, right click and then click on "Convert To Terrain". Once done you can then save your setup as a 3D Scene. Items such as trees, water or sky are automatically included in a saved 3D Scene.
When creating a new project to use the scene in you need to load it as the first item, otherwise everything present in the project will be deleted and replaced by it. Once added you can then start to build up your project using the 3D Scene as your base.
Gerry
System: Win 10 Pro (21H2), Asus X99-E WS, CPU i7-5930K -3,5 GHz, 32 GB DDR4 2666-16 RAM, NVidia GTX 1080 Ti GPU - 11 GB VRAM(Driver Studio-536.99), Samsung 850 Pro 512 GB SSD, 6 TB HD storage.