To turn on object snapping, use the Snap To Objects modifier for the Selection tool, or the Snap To Objects command in the View menu.
If the Snap To Objects modifier for the Selection tool is on, a small black ring appears under the pointer when you drag an element. The small ring changes to a larger ring when the object is within snapping distance of another object.
Turn object snapping on or off
Select View > Snapping >
Snap To Objects. A check mark appears next to the command when it
is on.
When you move or reshape an object, the position of the Selection tool on the object provides the reference point for the snap ring. For example, if you move a filled shape by dragging near its center, the center point snaps to other objects. This is particularly useful for snapping shapes to motion paths for animating.
Adjust object snapping tolerances