Usually multi rail PSUs just have a single 12V rail, which is split out to create the multiple rails, which should all have their own over current protections and bits. If yours is 4 x 40A, you can load up the 4 rails individually to 40A, provided the total load doesn't go over 1000W. So for example you could fully load 2 rails, with zero load on the other two, or load all of them up to around 20A each. You won't be able to load all of them to 40A, because they will all be driven by the single 1000W (~80A) rail.
I'd have thought that for a modular supply probably each PCIE output cable is driven by a separate rail, so if you are powering a 3080 from two separate cables, you'd be drawing somewhere around 150W / 12.5A on each at absolute full load, so well below the max. The 12v rail also powers the motherboard and CPU, so if you want to go to town balancing the load you could find out which rail the CPU / motherboard cables use, and make sure the PCIE cables are going to the other rails. I think this probably doesn't matter, unless you are trying powering the card from a single cable with a splitter, you aren't going to get close to the 40A limit per rail.
