Once the eShop loads, look for the Redeem Code option in the main menu. On the Switch 2, this appears in the left navigation ...

This cash can be spent on games (or DLC) via the Nintendo eShop and, unlike physical games, digital purchases don't require a cartridge, so can be taken anywhere easily. Although significant discounts ...

There's discounts on the other Nintendo gift cards too, so plenty of options depending on your budget. The best thing about these, aside from the discount, is that they're emailed to you, so no need ...