The designator ancestors returns a List of all notes in a direct line of ascent from the current note to its root-level container and so the List is returned in descending $OutlineOrder.
As the list of ancestors is returned in reverse order, i.e. deepest nested first and root item last. So:
$MyString = collect(ancestors,$Path)[-1];
is the root-level container for this note
$MyString = collect(ancestors,$Path)[0];
is the parent container for this note
See also—notes linking to here: