There are many ways to do this. For what I think you want to do, in the assemble command, change the FlashFit option to Connect in order to snap to keypoints- this will let you select centerpoints, edge points, etc...on both components. You can also snap coordinate systems together with Match Coordinate Systems, if your components have been designed to a common coordinate system.
EDIT: I believe I misread your question. Using the Include command in the sketch environment will include elements from other parts. If you want these elements to update if you change the part you included them from, make sure that "Maintain associativity when including geometry from other parts in the assembly" is checked in the Include Options dialog box. In addition, when placing parallel planes and other things in your part after creating-in-place, you can hold the SHIFT key to snap to keypoints in the assembly environment.
The geometry should have a little S shape for a relationship handle if it has been linked, meaning it will stay associative to the part it has been included from. If you do not see this handle on the geometry, it will not update when you change the parent part.
Placing planes using existing geometry is one of my gripes- it is not as easy as it should be. Planes in a part can be created coincident to planes created at the assembly level by once again holding SHIFT, but it is NOT easy to get at planes in another part document.
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