Either point a camera at the screen or buy a capture card.
You can also use the Homebrew called sysdvr but that requires you hacking your switch. For more information on that go to https://github.com/exelix11/SysDVR/releases. However be aware that hacking your switch can theoretically lead to your switch being broken. If you decide to buy a capture card, you don't have to go with a high priced one. You can just look up "720p capture card" or something and buy the cheapest option. The one I used only cost like 15€ (www.amazon.de/-/en/Capture-Camcorder-Support-Broadcast-Streaming/dp/B08BKNJP7N/) but you would need a HDMI splitter becasue the input lag is to high with the capture card i use this one: https://www.amazon.de/-/en/Splitter-Outputs-Amplifiers-Support-1080P-black/dp/B08HCLQV2V/ For setting up a capture card I recommend this tutorial: . He uses a different capture card but the setup is always the same.
