Right, so "f" and "i" are two separate grapheme clusters (and always are), but might map to one rendered glyph under certain fonts that combine them into a liga

Right, so "f" and "i" are two separate grapheme clusters (and always are), but might map to one rendered glyph under certain fonts that combine them into a ligature. Grapheme clusters have a specific definition that doesn't depend on the font in use. That definition is updated over time as the Unicode standard is updated, which means it can vary over time, but it does not change based on the font.