The discussion in this thread is misleading with regard to SketchUp layers.
SketchUp does not have geometric layers.
Therefore, geometry cannot “belong” to any layer, nor be put “on” any layer.
SketchUp “layers” are shared display property sheets, that can be “associated” with geometry to control display behavior, (in a sort of display sets,) across geometric collections (groups and components.)
The geometric entity’s “layer” is just a property that can hold a pointer to (currently only one) of these shared display property sheets, that are mis-called a “layer” in SketchUp. These “layers” can be associated with any number of dimensions, section planes, groups or component instances. (As Dave said, the primitive entitiy types need to be associated with “Layer0”.)