Private by default
Directories are private. Everything in one is reachable only by its owner and its members. A directory with no explicit visibility setting is private — missing never means public.
Sharing individual docs
The usual way to show someone something is to publish that doc. See Sharing a doc.
Publishing a directory
A directory can be marked public, which makes its documents readable without an account. This is how the documentation you are reading is served.
This is a deliberate, explicit setting. It is not inherited, and it cannot be set by accident.
Warning
Publishing a directory makes every document in it readable by anyone with the address, including anything a run wrote there. Check the directory's contents before publishing, not after.