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The outer perimeter of “silver bumps” aka solder balls is most likely all connected to ground. The rest must be signals. There is not enough cabling to support the required amount of signals between the two boards.
The interposer is still attached to the half in the top of the picture. The border of solder balls is higher than the rest of the PCB. This is required due to the night of the components on each PCB.
Truly incredible design. Passes data without cabling, affixes the two boards to each other without hardware, and creates an EMI shield without extra metal