https://www.gamespot.com/articles/mario-kart-8-switch-update-out-now-adds-video-capt/1100-6454200/

Coinciding with Nintendo Switch's big 4.0.0 firmware update, Nintendo has released a new patch for Mario Kart 8 Deluxe. The 1.3.0 update adds the ability to capture footage and makes a couple of gameplay tweaks, including to the dreaded Blue Shell.

One of the biggest features introduced in Switch's new firmware update is the ability to record and save videos of your gameplay. With the 1.3.0 patch, Mario Kart 8 Deluxe is one of the four titles to support the functionality thus far (the other three being The Legend of Zelda: Breath of the Wild, Arms, and Splatoon 2). Once the update has been installed, you'll be able to save up to 30 seconds of footage by holding the Capture button.