How can I make emacs rendering engine highlight a string of characters ?

[gnuist006]

How can I make emacs rendering engine highlight a string of
characters ? What are the emacs primitives to highlight, italicize,
bold-font, or such ? Is it correct that the text file is pure text and
emacs renders it according to the settings of its rendering engine? So
what are the primitives of this rendering engine ? Where do we find
other types of primitives ? These primitives are separate from the
lisp portion and they are probably what one would call the graphics
part of the emacs software.

Thanks a lot to the stars who can explain.

gnuist