From bash manual
Words of the form
$'string'are treated specially. The word expands to string, with backslash-escaped characters replaced as specified by the ANSI C standard
A double-quoted string preceded by a dollar sign (‘$’) will cause the string to be translated according to the current locale. If the current locale is C or POSIX, the dollar sign is ignored. If the string is translated and replaced, the replacement is double-quoted.
I wonder when the kinds of expansions happen?
I guess they must happen after parsing. If that is correct, when do they happen with respect to shell expansions such as brace expansion, parameter expansion, and filename expansion?