I see a lot of people talk about homemanager, but I've been happy enough with the following in my configuration.nix:

    system.activationScripts = {
      dotfiles = ''
        cd /home/chriswarbo/.dotfiles || exit 1
        for X in *
        do
          [[ -h "/home/chriswarbo/.$X" ]] && continue
          [[ -e "/home/chriswarbo/.$X" ]] && {
            echo "WARNING: Found ~/.$X but it's not a symlink" 1>&2
            continue
          }
          (cd /home/chriswarbo && ln -s .dotfiles/"$X" ."$X")
        done
      '';
    };

I've been using NixOS for many years, but my dotfiles repo goes back even further ;)