Architecture and disk question

Is proper support for arm64 included in nixpkgs, and also is there some native / built in way to declaratively create disk layout?

Not sure what you are expecting by “native / built in”, but there is GitHub - nix-community/disko: Format disks with nix-config [maintainer=@Lassulus] .

Included in nixpkgs is what I mean

Yes, but there’s no guarantee on the software support.

Not really.