In the simple example shown below, I'll edit the green part to add a hole that will be linked to the hole in the red part.
First I'll click on the green part (either in the graphics area, or in the tree), and choose the "Edit Part" icon. You could also choose "Edit Component" from the Assembly tab of the Command Manager.
The text for the selected Part will turn blue instead of black in the Feature Manager tree. You are now free to edit the Part within the Assembly just like you would in the Part file, and you can reference features from other Parts just like you would if they belonged to the Part being edited. Select the "Edit Component" icon in the Command Manager to exit the "Edit Part" function when you're done.
Now the green part references the red part, and any edits to the red part will be reflected in the green one.
As you can see, the hole was near the top of the blocks but is at the center in the last screenshot. I edited the hole in the red part, and the hole in the green part updated without any input from me.