How does Unity decide to render some objects over others? (2D)

I'm new to Unity, and I'm designing a small 2D project. I've got some objects in the scene, but I would like certain objects to be drawn over others (in other words, they aren't supposed to collide). I'm aware that I can put these objects into different sorting layers, but I'm wondering if there's an alternative method to render objects over others (does their position in the hierarchy matter?), as I would like to avoid creating a whole bunch of layers.

If someone could also direct me somewhither that explains how Unity decides the order in which to render objects in a scene, that would be much appreciated!