a huge amount of emoji are embedded png in font files. newer ones, however, do require more work.

> figure out which glyphs are required for your codepoint and use your font metrics to composite the glyphs appropriately.

this is all done in shaping though, the renderers just see "put this thing there" and (at least in GTK) we just maintain texture atlases that are updated on the fly.

you're correct that it doesn't solve the same glyph at lots of scales though. but glyphy does (and at high FPS too).