Doesn't it have those characters via extended ASCII? I seem to recall making boxes with characters back in my BASIC class.

"Extended ASCII" is just a sloppy term for a bunch of other encodings that are not, in fact, ASCII.

If your BASIC class used (or emulated) a C64 or compatible, you were using https://en.wikipedia.org/wiki/PETSCII and if it used MS-DOC you were using https://en.wikipedia.org/wiki/Codepage_437