You can convert videos to a format the camera app accepts. https://3dbrew.org/wiki/Nintendo_3DS_Cameraio mean file not web
Wrong.Doesn't work without hacking.
A stupidly odd workaround for the Switch without hacking it is to make a youtube account and upload your desired videos as private, then open the youtube app on the Switch, log into your newly created account, go to your videos and voila, you can watch them anytime and anywhere as long as you have internet connection. It's mandatory to have them as private, specially if these have copyrighted content, otherwise they will be deleted.