Convert video

MP4 to MKV

ffmpeg -i input.mp4 -c:v copy -c:a copy output.mkv

ISO to MKV

ffmpeg -fflags +genpts -i input.iso -c:v copy -c:a copy output.mkv

To get a better quality, you can use the MakeMKV tool, available on https://www.makemkv.com/.