Step by step: add a Nintendo eShop code to your Nintendo Account on the console or on the website.
A Nintendo eShop card adds funds to your Nintendo Account. You can use the funds for games, DLC and Nintendo Switch Online.
Sign in to the Nintendo eShop website for your country, open the option to redeem a code, and enter it. The funds are linked to your Nintendo Account, so they appear on the console too.
Nintendo eShop funds belong to your Nintendo Account, not to one console. Sign in with the same account on a new console to use them.