Separate download codes don't work if they allow users to resell the other physical card and keep the console version.
Have a physical card for the handheld. The home console has a card slot and can play an uprezzed version of the handheld game using the same assets. If you plug in an external USB drive, you also have the option to download additional assets to improve texture quality and possibly models etc. You can't play these without the card in the slot (so if you sell it, you can't play it on the console).
Nintendo could provide built in storage, but by making the home console work directly with the handheld carts, they save money by not needing a large HDD built in and save by not needing an optical drive. Keeps the home console compact, cheaper to build, and solid state so easier to cost reduce over time (optical drives and mechanical HDDs have pretty much bottomed out and are fixed costs these days)
(I think if they went with an optical disc format for the home console they would need a HDD for caching, because the shared architecture with the solid state handheld would mean developers wouldn't want to work around load so the home system would need a large cache. If the home console can read the handheld carts directly that cache wouldn't be needed)
Have a physical card for the handheld. The home console has a card slot and can play an uprezzed version of the handheld game using the same assets. If you plug in an external USB drive, you also have the option to download additional assets to improve texture quality and possibly models etc. You can't play these without the card in the slot (so if you sell it, you can't play it on the console).
Nintendo could provide built in storage, but by making the home console work directly with the handheld carts, they save money by not needing a large HDD built in and save by not needing an optical drive. Keeps the home console compact, cheaper to build, and solid state so easier to cost reduce over time (optical drives and mechanical HDDs have pretty much bottomed out and are fixed costs these days)
(I think if they went with an optical disc format for the home console they would need a HDD for caching, because the shared architecture with the solid state handheld would mean developers wouldn't want to work around load so the home system would need a large cache. If the home console can read the handheld carts directly that cache wouldn't be needed)