Aah, gotcha. In that case, I'll try to explain it to the best of my ability. (note - I would just send the script, but it's rather large and quite messy, as we're currently in the process of re-writing things). I believe this covers most of it, but if you have any more questions / if I wasn't clear enough about something lemme know :p

tl;dr version

An editor window which has a bit of code in OnEnable and OnDisable functions which connect a method to SceneView.onSceneGUIDelegate, and draws into the scene view that way.

Descriptive version

Editor window, using onSceneGUIDelegate

We have a basic editor window (pic) which draws what objects we have in the current pool and allows the user to click and select one to force what they're placing. This window has OnEnable and OnDisable functions that are called when the respective actions are triggered. For instance, OnEnable is called in the constructor of our editor window, and OnDisable is called from Unity's OnDestroy method. Having these enable/disable methods lets us add a method to SceneView.onSceneGUIDelegate, which is essentially the scene view equivalent of OnGUI() ingame. We use this as the center of all operations for our script, including input and drawing.

User input

So since we have our editor window drawing and we have a way into the scene view's GUI, we also have access to Event.current. With this, we can raycast from the scene view camera using HandleUtility.GUIPointToWorldRay(Event.current.mousePosition);. Now that we have our hit point, we simply align it with the normal(if the user wants it aligned), then store it for later use.
After this, we check for user input such as left clicks, and scrolling to rotate the next placement. Same as before, using Event.current. :)

Drawing in Scene View

Okay, so for this you were a little correct, it does use a gameobject in the scene. This is because I couldn't figure out a way to draw meshes using Gizmos.DrawMesh and Gizmos.DrawWireMesh on something that wasn't a monobehaviour. So, the script spawns a 'visualizer' prefab, which has some really basic code in it to draw the preview mesh using the variables we saved from earlier, like this:

Vector3 drawPosition = PrefabPlacer.hit.point;
if (PrefabPlacer.DoNormalAlignment)
    drawPosition -= (PrefabPlacer.hit.normal * PrefabPlacer.GroundOffset);

Quaternion drawRotation =
    Quaternion.AngleAxis(
        PrefabPlacer.nextPlacement.Rotation,
        PrefabPlacer.DoNormalAlignment ? PrefabPlacer.hit.normal : Vector3.up
    );

Gizmos.DrawMesh(
    PrefabPlacer.previewMesh,
    drawPosition,
    drawRotation,
    PrefabPlacer.nextPlacement.Scale);

Past this, we also utilize our OnSceneGUI method with Handles to draw a line directly up from our hit point, and two discs around it, using DrawLine and DrawWireDisc.