It's a basic tenet of Sketchup that to separate geometry you must either separately group the geometry or make it a component and then place instances of it. By default all geometry sticks together inside a model, or group or component. Hiding geometry or putting it onto layers that are switched 'off' will never separate the geometry.
Layers in Sketchup are NOT the same as layers in CAD. Layers are used to control visibility NOT geometry interaction.
Let's imagine a simple building. You'll probably want the walls on the 'WALL' layer and the roof on the 'ROOF' layer. That way you can switch off the roof and see inside the space from above...
To put things onto these layers so you can switch them on and off you should make the wall geometry with everything on Layer0 and then group the parts, make that group's layer WALL. Similarly make the roof geometry on Layer0 and group it [note how you can separate stuff early on - draw a couple of the roof's edges and group them immediately, edit that group and then everything you make inside the roof-group remains separated from the rest of the model, whilst you can still snap to any of it, drawn over it etc]. Now put the roof on layer ROOF. Now you can hide the roof or wall by simply switching off layers.
Also naming the roof-group and wall-group logically allows you to select these in the Outliner...
Putting the model's raw geometry [or basic geometry inside a group/component] will only stop it being seen - it will not stop it interacting with other things. It is usually best to have all raw faces and edges on Layer0 - keep the active_layer Layer0 [radio button], the visibility of layers is controlled by the tick-box...