Not exactly sure what’s going on without more detail, but in general you have 2 drains and a return line. So assuming the pipes are in the correct holes in the overflow box, you need 2 pipes going from the bottom of the tank to the sump, and one going from the return pump to the return line. The drain pipes typically will be 1 as your primary drain, and 1 as an emergency. The one with the black square will be your primary, as it’s a gate valve that lets you tune the flow. I’d guess the white is the secondary/emergency drain. The redsea site should be able to show you the as designed configuration, then you can see if the previous owner made any changes.
That union (last picture) looks like where someone hard plumbed a pump into the union, then used a barb fitting on the other side to attach it to soft tubing. The union is helpful for disconnecting the pump from the hard plumbing if it needs to be serviced.
