INCLUDE is to do with source files - it is the equivalent of #include in 'C'; it effectively 'pastes' the entire text of one file verbatim into another

EXTERN and PUBLIC are to do with symbols:

PUBLIC lets other modules "see" a symbol from the defining module;

EXTERN is used in the other module(s) to say, "this comes from somewhere else"

The Linker "joins-up" the EXTERN references to their PUBLIC definitions or, if it can't, gives you the "reference to undefined symbol" error!