When you create a new mount namespace, the kernel creates a copy of every mount reachable from the filesystem (as visible to the process creating it). Then, it

When you create a new mount namespace, the kernel creates a copy of every mount reachable from the filesystem (as visible to the process creating it). Then, it changes the current process's root directory and current directory to point to the same paths in the corresponding mounts. Changing a process's root via chroot is otherwise a totally separate operation.