ML-TT Ambili is a legacy (non-Unicode) font. Instead of storing real Malayalam characters, it draws Malayalam shapes on top of English keys, so a document typed in it looks like random Latin letters on any device that doesn’t have the font installed.
Converting to Unicode stores the real characters, which every phone, browser and search engine understands. This converter’s mapping is based on a published open-source table; the Sources list at the end of this page names it and its licence, and the chart below lists every mapping the converter uses.