Smash Replay. After the match is over, press Y to save the video(it says so in the bottom right hand corner.) After that, in the replay video section in the vault, you can convert it to a video and then once that's done, you have to THEN plug take the Micro SD card out of the Switch and plug it up to the computer and in some folder directory there will be an MP4 file.
Nintendo oughta be shame of themselves for this shyt