Yes `nix-env -iA` is installing packages in an imperative way. I think it is there to be some kind of tool that people from other OS can relate to. Purist say you should avoid using it for installing packages and instead list global packages in `/etc/nixos/configuration.nix` for globally installed packages and home-manager for user specific ones, and if you need temporarily just to try something out use `nix-shell -p <whatever>`.
Back to your second question, you can configure the system through `/etc/nixos/configuration.nix` it is enough to configure system as a service. Pretty much everything you could do through Chef/Puppet/Saltstack/Ansible/CFEngine etc.
home-manager is taking it a step further and do this kind of configuration per user. It is actually written in a way that can be added to NixOS (or nix-darwin for OS X users) to integrate with the main configuration so then when you're declaring users you can also provide a configuration for each of them.
So it all depends what you want to do, the main configuration.nix is good enough if your machine to run specific service, that's pretty much all you need, you don't care about each user configuration in that scenario, you just create users and start services using them.
If you have a workstation, home-manager while not essential can be used to take care of setting up your individual user settings, stuff like dot-files (although it goes beyond that). The benefit of using home-manager is that most of what you configure in it should be reusable on OS X as well.
If you care about local development, you can use Nix to declare what is needed, for example[1]. This is especially awesome if you have direnv + lorri installed
you can add these to home-manager configuration:
programs.direnv.enable = true;
services.lorri.enable = true;
When you do that you magically will get your CDE (that includes all needed tools, in this case proper python version, you also enter equivalent of virtualenv with all dependencies installed and extra tools) by just entering the directory, if you don't have them installed all you have to do is just call `nix-shell`.
I also can't wait when Flakes[2] get merged. This will standardize setup like this and enable other possibilities.
[1] https://github.com/takeda/example_python_project
[2] https://www.tweag.io/blog/2020-05-25-flakes/
