How to convert a video file in format as flv to mp4 with ffmpeg?
You may use method from here: https://www.systutorials.com/3540/converting-video-files-for-ipod-iphone-ipad/
ffmpeg -i input.flv output.mp4
Or by specifying more details about how to handling the codecs:
ffmpeg -i input.flv -vcodec copy -acodec copy output.mp4
You may also add more options as you like. For more, check the ffmpeg manual: https://www.systutorials.com/docs/linux/man/1-ffmpeg/