Unicodestrings (used to represent unicode character strings) are implemented in much the same way as ansistrings: reference counted, null-terminated arrays, only they are implemented as arrays of WideChars instead of regular Chars. A WideChar is a two-byte character (an element of a DBCS: Double Byte Character Set). Mostly the same rules apply for WideStrings as for AnsiStrings. The compiler transparantly converts WideStrings to AnsiStrings and vice versa.
Similarly to the typecast of an Ansistring to a PChar null-terminated array of characters, a UnicodeString can be converted to a PUnicodeChar null-terminated array of characters. Note that the PUnicodeChar array is terminated by 2 null bytes instead of 1, so a typecast to a pchar is not automatic.
The compiler itself provides no support for any conversion from Unicode to ansistrings or vice versa. The system unit has a unicodestring manager record, which can be initialized with some OS-specific unicode handling routines. For more information, see the system unit reference.