How much meat is there?
-Enough
Where are the fluid passages?
-The path is only on the one side with the pistons for the rears. You wouldn't see anything with it split apart.
The fluid enters through the brake line and enters a void behind the pistons. They most likely have a narrow single channel with an o-ring that jumps to the other side for the fronts. I would assume they are separate since we commonly get just one side seizing.
There is also a small pass for the air valve to enable the bleed procedure.
Everything else is solid. It is a cast part so the fact that it is two pieces would typically just be to simplify the mfg process.
Edit* I was able to find this image of a similar setup. looks like just one crossover passage*
