The sump is fed by gravity from the tank overflow (either a drilled bottom, back or a hang on overflow box). The amount of water flowing from the tank to the sump is determined by the size of the hole and pipe. The protein skimmer is fed by a pump (usually a submersible in the sump, but it doesn't have to be). The outflow from the PS is usually a large open section of pvc
for an in-sump PS, or another piece of pvc
from the PS back to the sump. A refugium can be a seperate container or just a section of the sump where you add macro algae. The return pump is in the last section of the sup (usually, but some have it in the middle). The return pump must be sized to the flow rate of the overflow. If you have a 1" overflow then it is capable of feeding 600 gph
to your sump. You want a return pump that will produce 600 gph
or LESS at the head height of the return pipe outflow.
I know, clear as mud....