As brazzy said, there's no such thing as extended ASCII. There's just a huge number of ASCII-compatible eight-bit encodings. The original IBM (and DOS) character set, hardwired into ROM, is the one you're thinking of, and went by various names such as "Personal Computer, MS-DOS United States, MS-DOS Latin US, OEM United States, DOS Extended ASCII (United States), PC-ASCII" [1].

DOS 3.3, in 1987, was the first version to support localized character sets, via a system of "code pages". You'd select an encoding/"character set" that suits your language in AUTOEXEC.BAT – or just used the default 437 if you were a US user and never had to worry about these things. For me, the most relevant code page was 850, aka "OEM Multilingual Latin 1" (not at all the same as ISO/IEC 8859-1 which is also known as "Latin 1").

Why the apparently arbitrary numbers, I'm not sure, but Claude and ChatGPT both claim the codes were simply drawn from a more general-purpose sequence of product numbers used at IBM at the time.

This application, like other similar ones, uses Unicode box drawing characters that now all reside comfortably out of the eight-bit range.

[1] https://www.aivosto.com/articles/charsets-codepages-dos.html

"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