I've been making 2D games for a very long time, but I'd like to start dabbling in 3D. It's surprisingly difficult to find an answer or tutorial on the Internet for creating custom 3D shapes for SceneKit.
What editors support the appropriate file type? Would you suggest a particular one?
I personally use Cinema 4D, exporting as Collada 1.4 (.dae), but you can also use Maya, 3DSMax, or Blender.
They all have different styles: C4D is the easiest to learn, Blender is free, Maya is the standard for games and 3DSMax for architecture.
They all have extensive getting started and advanced tutorials and courses online, so just look around and pick the one you prefer!