How does the PostgreSQL server communicate with the database?

The PostgreSQL server communicates with the database through internal processes and the file system. The server itself is a set of processes that manage access to data stored in files on disk.

When a client connects to the server, a separate process is created to handle this connection. This process interacts with the database files, using a buffer cache to improve performance. The server processes SQL queries, plans their execution, reads and writes data to files, and also manages transactions and locks.

Thus, PostgreSQL is a server that directly works with database files and provides multi-user access, consistency, and data integrity.