If you mean the general principle of this kind of pseudo-GUI in text mode, look into codepage 437 [1]. It's possible to do more advanced stuff with custom glyphs [3], but it doesn't look like this does anything like that - it generally requires direct hardware access to modify the font.

This was a pretty common technique in 90s DOS programs, but it can be tricky to get working cross platform over a serial terminal. The library you found seems to rely on termbox [2] for that.

[1] https://en.m.wikipedia.org/wiki/Code_page_437

[2] https://github.com/nsf/termbox-go

(edit)

[3] I thought Norton Commander did this, but I was mistaken - it just uses codepage 437. It was Norton Utilities [4].

[4] https://en.m.wikipedia.org/wiki/VGA-compatible_text_mode