A couple of hopefully helpful tips based on my relatively limited Godot 3D (& non-3D modeller) experience:
- CSG ("Constructive Solid Geometry") nodes (which enable you to create complex shapes by combining simple shapes together with boolean operations) can be a great way to start prototyping 3D games/levels but be aware that there are some trade-offs involved in terms of the "quality" of the resulting mesh. This is primarily a factor when you want to start applying textures & you may encounter issues with UV coordinates/normals etc.
There's a CSG intro tutorial here: https://docs.godotengine.org/en/3.3/tutorials/3d/csg_tools.html
You may wish to read-up on some of the potential issues (e.g. here: https://github.com/godotengine/godot/search?q=csg+uv&type=issues) before committing to CSG in case your plans might be impacted... - In part I started prototyping with CSG because I wasn't aware that there was another option for easily creating simple a.k.a. "primitive" (e.g. cube-based) level geometry in Godot (an option which also has the benefit of seemingly having fewer issues with mesh quality/UVs etc).
The prototyping option I now turn to first, is to add a MeshInstance node to the 3D scene and then click on the `Mesh` property which enables you to choose to instance one of a number of simple/primitive meshes, e.g. cube, capsule, cylinder, etc.
Hopefully that helps some people avoid running into the same issues I did. :)
Bonus tip: If you're looking for some basic prototype textures to apply to your prototype geometry the famous "Kenney" has some freely downloadable CC0 (public domain) textures available here: https://kenney.nl/assets/prototype-textures. (And also directly through the Godot Asset Library on the web or in the editor: https://godotengine.org/asset-library/asset/781.) (And, someone who, just quietly ;) has also recently started using Godot for some of his projects. :D )