Host identity and topology live in modules/hosts/<host>/default.nix; shared account, service, external-device, and server-topology facts live in modules/facts.nix.

Add a package used by both workstations to the relevant Home Manager capability, or to workstation-apps in modules/capabilities/desktop-defaults.nix when it needs no configuration.
Add a host-specific capability to that host's import list in modules/hosts/<host>/default.nix.
Change the selected terminal or editor by replacing the corresponding primary-terminal-* import in modules/profiles/workstation-base.nix or primary-editor-* import in modules/profiles/host-base.nix.
Keep native theme settings with the application. Explicit Kanagawa color mappings may import modules/capabilities/_kanagawa.nix.

Common commands:

nix fmt
nix flake check --no-build
nh os switch
sops edit modules/secrets/secrets.yaml