Nothing ship a Glyph Matrix SDK for the Phone 3 and a debug flag that has to be
set over adb before the phone will load an unsigned toy. That is most of the
work: the display is a small monochrome grid, and drawing legible digits on it is
a typography problem more than a rendering one.
The rest is a background service, a five-minute refresh, and a long-press handler that toggles between the icon and the number.
It only runs on the Phone 3. Every other Nothing handset — Phone 1, 2, 2a, 3a — has the older glyph strips rather than the matrix, and there is no graceful degradation to write.