Here’s how it works – you login to your Nintendo Account on the official Nintendo website, find a 3DS or Wii U game that you want to view on your Nintendo Switch, and put that/those games on your wishlist.
Once you’ve done that, open the Nintendo eShop on your Nintendo Switch, find your wishlist, and voila, you can view those games on the Nintendo Switch now.
(as a reminder, games I put on my Wishlist on Nintendo's official website all show up on the Switch eShop, even 3DS and Wii U games) pic.twitter.com/jJF2gItvBv