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This 9v battery contained six cells stacked like a layer cake
(lemmy.sdf.org)
Picture of a disassembled Duracell 9v battery. Below the terminal assembly is a clear plastic case where you can see six sets of stacked rectangular terminals and fillings.
Cheap low-capacity 9V batteries are still 6 AAAA cells. The flat cells allow higher capacity in the same space, so you find them in the batteries that advertise themselves as long-lasting.