I don't know how to explain it any simpler. Iterating over a str type in Python 3 enumerates Unicode code points. The length of a str type is the number of code points it contains. Reversing a str will reverse the Unicode code points it contains (not guaranteed to be a sane operation). Indexing into a str with foo[0] gives you back a str containing a single Unicode code point.

This is not an implementation detail, it is fundamental to how the str type in Python 3 operates. I have not talked at any point about the internal storage of this type, just the interface it publicly exposes.