I think it is mentioned there: https://int10h.org/blog/2015/04/cga-in-1024-colors-new-mode-...

In short, textmode gives you 80x25 characters of 8x8 pixel resolution each, making 640x200 resolution, with 16-colours, where each character can be assigned a foreground and background colour. The 'Macrocom Method' reprograms the character height from 8 rows to 2 rows, which effectively gives you 80x100 characters. You can then use each of the 256 characters in the characterset as an 8x2 'glyph' with a foreground and background colour to build your images from. So you can't technically control the colour of every pixel on screen, but you can make a decent approximation with well-chosen character 'glyphs' and foreground/background colours.

This can also be automated with pattern-matching, where you convert an image to glyphs by algorithmically selecting the nearest match.