Info:

nixos-rebuild switch = build the new system configuration (from /etc/nixos/configuration.nix or flakes) and activate it immediately without reboot.

• It builds the new system in /nix/store.
• Updates the GRUB/systemd-boot entries.
• Switches to the new configuration right away (systemd units, services, kernel params if possible, etc.).
• Leaves the old generations available for rollback (nixos-rebuild switch --rollback).

So: “switch” means build + activate now

–impure

nix flake update
sudo nixos-rebuild switch --flake .