Because of the height, you have to put the quadratic one onto the BCM2712 SoC (silver), and the square (with the longer edges) onto the memory chip (in your picture just above the BCM2712 SoC, with the M marked.
For users who update: Your thermal pads might be dried out or just broken otherwise. I’ve run some tests with and without thermal pads and CM5, in my testing it seems that makes a very minimal difference. This are tests under full load:
It seems that the rather large and flat aluminum surface of the BCM2712 provides quite good heat transfer on its own. So operating without thermal pads should be totally fine too.