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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.
IIRC there's a transparent version of the Fairphone 5. Not that it's as aesthetically interesting inside as older tech, more of a statement about the device's modular design.
I just looked it up. It's a little too opaque for my tastes based on what I'm talking about but you're right, it's not as interesting inside. Maybe that's why other companies stopped doing it. It still makes sense if you want to show off the design of the device in a specific way like you said.