Install the nix-zsh-completions
package or see next comment.
programs.zsh.enable = true;
programs.zsh.enableCompletion = true;
There are a bunch of out of the box options for zsh. See: NixOS Search and https://github.com/NixOS/nixpkgs/blob/09aa1b23bb5f04dfc0ac306a379a464584fc8de7/nixos/modules/programs/zsh/zsh.nix
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