There is also https://nixery.dev/ which lets you get Docker images with arbitrary Nix packages - useful for quick hacking.
You use Nix the language to build Docker images and avoid the NixOS CLI tools/runtime?
Nix the language.
NixOS the Linux built with Nix as it's default package manager that uses the Nix language.
Or is that Nix the package manager? Does Nix the package manager compile Nix the language images?
No
> can you build OCI images using Nix from Mac/Windows/other non NixOS Linux?
Non NixOS Unix, not Windows yet, unless you use WSL or a VM