

It’s a mix of --codec copy to not reencode the streams, --map 0:1 to select the audio stream of the input file and for merging back you need multiple -i input files and then --map 0:0 to map the first input file’s video stream and --map 1 to map the second input file (English audio stream). Mappings may vary - use ffprobe to check your files.







The video I saw had a disclaimer at the end saying that no device has been built yet.
Reminds me of this one, that made rounds some years ago and turned out to be fake as well.