How Extensibility Works
PostgreSQL is extensible because its operation is catalog-driven.
PostgreSQL stores much more information in its catalogs: not only information about tables and columns, but also information about data types, functions, access methods, and so on. These tables can be modified by the user, and since PostgreSQL bases its operation on these tables, this means that PostgreSQL can be extended by users.
The PostgreSQL server can moreover incorporate user-written code into itself through dynamic loading.
