Sunfish suggest you study bgt namely GG and GH. V increase PX decrease temp decrease.
The scroll increase velocity into the turbine ( divergent duct ) iT moves across the turbine airfoil this creating rotation (ie lift ) then exits velocity decreases pressure increases temp increases.
Inlet side
Compressor opening is large into the comp it is compressed velocity is increased px decreased temp decreases as it passes through the scroll divergent now velocity decreases px increases temp increase
The compressed gas will only compress if the engine cannot use all of the flow that it is at it's use.
Simple basic gas.
Cheers