

The video is the same. The audio is adapted to the video.
.mkv containers can include many things, mp4 only one audio one video afaik
Using ffmpeg you can extract audio and video and combine them how you like into an mkv or whatever. Very powerful tool
Handbrake if you prefer learning a complex GUI
Docker to Podman, absolutely. Also checkout using base images instead of random intransparent downstream ones.
VMs are debatable. They are nice for separating things way better than docker does.
As distro, NixOS is very custom and you will want to learn its way. Arch does not make sense I think, but you can use anything as a server.
Otherwise Alma or Debian? Try HeliumOS too! Or uBlue uCore