While Godot is no substitute for Blender and similar modelling packages, you can in fact use it for prototyping your levels.

You can use Mesh nodes with a corresponding Collision shape for custom platforms, Godot has built-in Mesh shapes for boxes, cylinders and more.

You can also use the CSG nodes to build more advanced prototype levels, since these allow merging or subtracting individual shapes from each other.

There’s an excellent video on this on YouTube. It’s made for Godot 3, but the core concepts should also work for Godot 4: