I would like to allow the user to rotate the scene by touch but have the lighting remain fixed. This works quite well using the default camera and default lighting. However, the default light is "straight on", i.e. along the screen's -z axis. I would rather it be directed at an angle more like a stage light, say from the front upper right.
But when I create my own light it appears that it needs to be attached to an existing node, the rootNode for example. When this is done, the light then rotates around with the model as the user manipulates the scene.
Is there a simple way to keep the lighting fixed while rotating with the default camera or do I need to get seriously involved creating a custom camera?
The lighting is already "fixed": that is, each light source keeps its position and direction within the scene unless you do something to change it. But it sounds like instead, you want to have a light that is fixed relative to a camera.
To achieve this, don't attach the light to the scene's root node. Instead, attach it to the same node that the camera is attached to. Or if you want to adjust the light's position relative to the camera, you could construct a small node tree, with one leaf containing the camera and the other leaf containing a directional light.
You'll almost always want to create your own camera or cameras in SceneKit. The default user-manipulable camera is useful for quickly getting up and running, and debugging, but not something that you want to expose to end users.