Internal functions are functions written in C that have been statically linked into the Postgres backend process. The AS clause gives the C-language name of the function, which need not be the same as the name being declared for SQL use. (For reasons of backwards compatibility, an empty AS string is accepted as meaning that the C-language function name is the same as the SQL name.) Normally, all internal functions present in the backend are declared as SQL functions during database initialization, but a user could use CREATE FUNCTION to create additional alias names for an internal function.
Internal functions are declared in CREATE FUNCTION with language name internal.
Read the upstream documentation. For corrections, first check the current version.