You may want to read my blog post [1] on that one. It explains exactly how to install Sway with Home Manager.

And no, it is not a design error to have Home Manager separated from nixpkgs, and unlike a child reply suggests, there's no "schism", and the reasons for separation are rather prosaic, as explained in another blog post of mine [2].

[1] https://drakerossman.com/blog/wayland-on-nixos-confusion-con...

[2] https://drakerossman.com/blog/how-to-add-home-manager-to-nix...