If this is confusing, maybe it could be explained a little better. This part of the system requires an understanding of voltage and amperage, but it's fundamental in understanding how panels are wired. Every system with more than 2 strings require a combiner box, whether it's located at or within the inverter, or higher up near the array.
Benefits Of Combiner Boxes
Many installers use combiner boxes as an opportunity to change the wiring from outdoor-rated USE-2 wire, to the conduit-insulated THHN or similar, which can fit more comfortably in the EMT that is required. It's also a chance to fuse a system upon entry to a building, which is a smart move, from the perspective of fire safety.
Combiner Box Assembly