At a guess, I would say a cavity made up of insulation (glass wool), wiring, roof framework (battens) and the aluminium skin which flops around on top.
Taking the old ceiling out is easy, putting the new one in is a slightly different matter.
Make up some lengths of wood to fit between floor and ceiling to use as props for the new board, you will still need 15 hands.
Sort out the leak first otherwise your hard work will be wasted