Fortnite gift cards can be used to add funds to your Epic account balance so you can buy a wide variety of things, anywhere that uses Epic's payment system. This includes:
Fortnite V-Bucks, Fortnite Crew, and Packs
Epic Games Store games, DLCs, and add-ons
fab.com content (for creators)
Fortnite gift cards are currency-specific, so the currency of your card must match the currency of your Epic account balance to redeem the card.
If what you're purchasing costs more than what you had on your gift card, your account balance can be combined with other payment methods to complete your purchase. Split payment is not supported for subscriptions, including Fortnite Crew.
Nintendo devices don't support the Fortnite Shared Wallet feature, so to spend V-Bucks purchased using your account balance, you will need to go to the web Item Shop or use a Shared Wallet platform (that's connected to your Epic account) including:
PlayStation
Xbox
PC
Mobile
Cloud gaming services
Fortnite items which you purchase using V-Bucks are available on all platforms, including Nintendo devices.