Illustrator stacks successively drawn objects, beginning with the first object drawn.
How objects are stacked determines how they display when they overlap. The stacking order of objects depends on the drawing mode you are using. When a new layer is created in the Draw Normal mode, it is placed right above the active layer and any new objects are drawn at the top of active layer. However, in the Draw Behind mode, when a new layer is created, it is placed right below the active layer and any new objects are drawn behind the selected object (or at the bottom of the active layer if no objects are selected).
You can change the stacking order (also called the painting order) of objects in your artwork at any time using either the Layers panel or Object > Arrange commands.