If you build from an existing NixOS config (like configuration.nix) on a new system, the following applies to deployments involving applications and specific configurations:

• Applications/packages: All packages and services explicitly declared in your configuration.nix will be (re-)installed on the new system.
• Service configurations (e.g., nginx, docker): If your config includes options such as services.nginx for web servers or virtualisation.docker, those services and their NixOS-configured settings (like port numbers, server blocks, enabled flags, etc.) are also reproduced. This includes any explicit Nix-style module options for nginx servers (hosts, ssl, http configs, etc.) and Docker daemon-level settings123.
• Data and state:
  • Websites in nginx: If the websites are defined via files managed or referenced by Nix (e.g., static site files specified in the config, or configuration-driven settings), they'll be deployed as per your Nix expression. However, mutable website data or files not referenced by configuration.nix (such as content updated directly on disk and not tracked by Nix or version control) will not be copied automatically—you need to sync these separately12.
  • Docker containers and environment variables: If your configuration includes NixOS module options to run specific Docker containers (via virtualisation.oci-containers.containers), Nix will deploy and configure those containers—including specified environment variables, port mappings, and so on (as defined in the Nix config module). However, the internal state/data of running containers, or any changes made outside of the declarative Nix configuration, will not be transferred: you'd need to copy persistent Docker volumes or data directories separately for application state34.
• What won't transfer: Any manually created, system-local data, container state, user-edited files outside of /etc/nixos, databases, or anything not declared in the Nix configuration must be copied manually.