In addition to nesting, you can put text directly into *sidebar and it will show up outside of any card, directly on the page background.
To give everyone their own custom card in the sidebar, include {{_user+menu}} (nothing special about "menu", it could be anything). People who are signed in will be offered to "Add |your name|+menu". To prevent people who are not signed in from seeing "Add Anonymous+menu" there, go ahead and create Anonymous+menu and set its permissions so that only people who are signed in can see it.
To have things in the sidebar that are only visible to some people, add a role and assign each of them to it, then include a card in the sidebar with the private content and set it so it can only be seen by that role. See permissions.
You can change where the sidebar appears on a page (or whether it appears at all) — see custom layouts.
You can see any page without the sidebar by adding "?layout=noside" to the URL. (See web address for everything.)