How is unicode implemented behind the scenes?

[Dan Stromberg]

OK, I know that Unicode data is stored in an encoding on disk.

But how is it stored in RAM?

I realize I shouldn't write code that depends on any relevant
implementation details, but knowing some of the more common
implementation options would probably help build an intuition for
what's going on internally.

I've heard that characters are no longer all c bytes wide internally,
so is it sometimes utf-8?

Thanks.