Septic systems are plumbing systems. In this view, the concrete D-box has been placed where it belongs and the effluent lines are being pushed into the seals. Although all the other pipe joints are glued together, where the lines enter the tank and D-box, the pipes are pushed into the special seals without glue. Older methods would require lines to be sealed into D-boxes and tanks with concrete gout.
Distribution flow must be even and can be determined/test by pouring a bucket of water into the box and rotating the seals to make the opening in each seal break the surface at the same elevation. This evenly distributes the flow between the outlet lines leading to the trenches.