Expanding on what @mihai.s wrote:
It sounds like you may be coming to SketchUp with experience in 2D drawing or CAD. In those apps, the concept is that you are looking straight down at a stack of 2D objects in which the order in the stack determines which is “in front of” (obscures) which others. So you “send to back” to push something behind, to move it “down” or “back” in the stack.
But SketchUp is always 3D. You can orbit the camera around the model to look at it from any direction in 3D space even if the model is actually 2D (is drawn on a plane embedded in the 3D space). So, if you orbited around to look from the opposite direction, what would “in front of” mean then? The former front side is now the back! If you orbit to look at the plane edge-on, is left “front” or is right “front”. Because of these aspects, SketchUp has no concept of a viewing stack. Instead, whatever is closer to the camera blocks view of anything that is farther away and behind it. This relationship changes every time you orbit around the model.
The nearest you can come to “send to back” is to move an object in 3D space so that it is positioned behind another when you view from a particular direction. In your case you are looking down at the xy plane from the z direction, so if you move something down in z the ones nearer to the camera in z will obscure it, as @mihai.s suggested.
When applying @mihai.s advice about layers, please realize that SketchUp layers are also not like 2D drawing or CAD layers. SketchUp’s layers just provide visibility and color flags that multiple objects can share, layers do not “own”, organize, or isolate objects from each other. An edge or face “on” a non-visible layer will still intersect and merge with ones “on” visible layers. SketchUp groups and components gather geometry together in a sort of container that isolates it from such interaction.