It doesn’t use ordinary floats?

Unless my test methodology was flawed:

Displays 0.3:

"=0.1+0.2"

Displays 3E+79:

"=(0.1+0.2)*1e80"

Try "=((0.1+0.2)-0.3)*1e80" -- your test can be fooled by pretty standard rounding for display.

Oops -- nice, thank you! Sorry, guess I was mistaken

That could be just the way it's displaying, e.g. to 14sf or something. If you try adding a million 0.1s and use a formula to compare to 100,000, that might tell you (with floats I got 100000.00000133288).

This question suggests it uses floats: https://stackoverflow.com/questions/74542790/google-sheet-yi...