https://www.gamespot.com/articles/heres-how-to-play-nintendo-switchs-new-nes-games-o/1100-6461882/

Nintendo's online subscription service for Switch has arrived, and along with online play and cloud save backups, one of the benefits it provides is access to NES - Nintendo Switch Online, a library of classic NES games exclusively for subscribers. There are 20 titles available in the library right now, with more to come every month, and they all feature modern amenities such as save states, HD resolution, and even online play. If you're interested in trying out the latter, however, it may not be readily apparent how exactly to play NES games online with a friend.

If you subscribe to the Nintendo Switch Online service, you'll be able to download the NES app from the Switch Eshop or Nintendo's website for free. You'll have unlimited access to this app for as long as your remain a subscriber, and the NES games can be played offline for up to seven days, but you'll need to "check in" periodically to verify that you still have an active subscription.