Please keep in mind that only 3 things in SketchUp “own” geometric collections.
These are the top level model, and the definitions of components and groups.
(Whilst ignoring Image objects for this discussion.)
Layers are shared display behavioral properties that are used by entities.
So an entity has a setting that can point at one layer to use at a time.
(You’ll just confuse yourself if you continue to think of objects being on layers, or moving objects to layers. But you’ll also notice even the gurus and sages “slip up” in this regard.)
As said above, keep all geometric primitives set to use
"Layer0" and only assign group or component instances to use other layers.
There is a special case when primitive entities might need to be temporarily assigned to use a layer other than
"Layer0" and this is just before exporting out to another file type for use usually in a 3rd party CAD application. But the good exporters will do this for you and undo any change to your SketchUp model.