I have found something where the NixOS manual is the most accessible and readable of any resources I've found: declarative containers!
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Turns out that declarative containers are implemented as a pile of bash: https://github.com/NixOS/nixpkgs/blob/635c7ea3298176e57716e88ed3d820ce66766c91/nixos/modules/virtualisation/nixos-containers.nix
Which, in hindsight, I should've seen coming.
