There is controversy in the nix world because nix flakes were (some say) merged without proper vetting first. OG nix diehards don’t want to taint nix documentation with ‘experimental’ flakes. But probably the majority of nix users are all in on flakes. So you have documentation from the OG camp that doesn’t include flakes, and you have innumerable unofficial guides for the flakes way. This on top of the quirkiness of nix the language and the multiple ways to do things. Unfortunate.
IMO nix-env was a mistake. It feels like an imperative package manager which may be comfortable to new users who are used to apt or similar. But really what you should be doing on nix is maintaining *.nix files which document/specify your system setup, and nixos-rebuild to update your system to that configuration. Similarly, nix channels are an imperative holdover, which can be done away with if you use flakes, which results in your current nixpkgs version being documented in a system level flake.lock file.
Good Nix OS series: five articles for new users. (feddit.it)
I found these interesting Nix OS articles series for users that want to try it and learn first things, have a good reading!...