Hi,
The upper building fail to snap properly to the lower part and I can see there is Z fighting between those two parts. May I know why?
https://youtu.be/pPU8lvg1Oms
Hello Zulfariz. You have to position your mouse over the location of the desired vertex you want to snap to.
Try it with a simplified scene. Start a new Blender and duplicate your cube, then try snapping one cube to another and you'll get the hang of it. Remember, you mouse cursor has to be in the position over the vertex, edge or face you want to snap to.