The text read from the document is held as Unicode, and displayed using the built in fonts. By default these are the Google Noto fonts, which are in OpenType format. One of the features of OpenType fonts is the ability to offer excellent typographical output for a wide range of scripts due to the inbuilt automated tables to control font shaping.
Font shaping allows a font to choose a different set of output glyphs (with highly customised positioning) based on the context within which an input Unicode character (or series of characters) are used.
Some languages use this to add diacritical marks (in particular Vietnamese). Others (such as Arabic) may use it to ensure that characters join smoothly. Still others (such as Indic languages) completely change the appearance of groups of input characters by combining them into single shapes that represent multiple characters at once.
The complex rules that control this are encoded as tables within the OpenType format fonts. The interpretation and application of these tables/rules is handled for us using the HarfBuzz library.