It would be the exact same amount of effort you'd use to get new software on other distros. Both Arch and NixOS have very straightforward methods of installing new software that aren't any more difficult than doing so on Debian or some other distro. Both Arch and NixOS support independent package managers like flatpak and snap + they support Appimages.
I'd also add that OP doesn't even need to use NixOS to use nix packages, whereas Arch or Debian would require systems based on those distros. So if anything NixOS tries to make it very easy to add and configure software. Where does all the effort come in?
No that's "predestination". You're thinking of the process by which carbohydrates are synthesized from carbon dioxide and a source of hydrogen, using light as an energy source.