Help with my sump

[Rcraft]

Okay so I am trying to get my phone hooked up to a 125 gallon tank I have been watching videos and reading and nothing is telling me what goes into the sump I know that my tank water drains into the first section goes into the second section which if I'm understanding everything right is a refugium ( I did learn that I do put sand or live rock) then it goes into the next section which I believe is where I put my protein skimmer and the pump to it back into the tank I am Not sure what goes into the first section do I put bio balls in there or do I put on some kind of a filter in there. My tank have 2 corner overflows with 2 1 inch holes and 2 3/4 hole (I believe are the returns lines) if someone can help me please
pictures of my sump

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[roadie]

1" lines drain 3/4 is for return , mine goes thru a filter sock then I have skimmer and heater in center . My sump is gen three eshops so it is a little different than this one but same principles