It assembles and pivots to own root filesystem. The filesystem mounts dirs and files from your original filesystem, most of them in read-only mode, but does not mount files that expose secrets to the sandbox. The sandbox also mounts own, writable version of your home dir, so secrets, such as .ssh dir are not exposed. The config file allows to configure which files should be exposed from your original home. For example, configs, such as .bashrc, are safe to expose in read-only mode, provided you don't store secrets in them.