Nix is used for app deployments?

Nothing stops people from using it that way.

I would kind of prefer appimages / flatpak's though ?

I think appimages are the best way if we can get aside from the fact of some limitiations it has if I remember like if you build the project on X then code can only run on linux versions on Y where there is some relation b/w X and Y , I know its very vague... It was some reddit post.

Man , I have watched / read so many posts that I only vaguely remember things but I really don't have exact bookmarks and it just feels so repetitive and kind of humiliating to say these again and again....

If you are using Nixpacks, then yes it is used to build you docker image.