Databases are not that easy.. they are very advanced in terms of storage etc.

They may be having their own file system and/or store data in pages. I know of Sybase that can be installed even on a raw disk (but having multiple databases on the same raw disk makes it less efficient).

Also, it is not necessary that every write or read operation would take places from files.. they could be in-memory transactions as well that the database engine periodically writes back to its storage.

You should be better asking this question on a good database forum or a book/article telling about that.