It's inside the big white square thing. Show a pic of the full width of the cistern.
To get to it you will most likely need to seperate the cistern from the pan ,after isolating and removing the water from the cistern. That's just for starters !!!
A diaphragm mechanism can be / or be swapped for one that allows diaphragm replacement by slipping a pin out of the full cistern without draining. Well worth the effort in swapping if you have to disassemble anyway
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