How are "DEFAULT PRIVILEGES" implemented in Postgresql?
information_schema is an ANSI-standard set of read-only views. You can easily learn what that views actual query are by reading the code which is here. The real catalog information is stored in System Catalogs.

As documentation says 'There is no ALTER DEFAULT PRIVILEGES statement in the SQL standard.' So there shouldn't be anything related to information_schema. 

Your records should be stored in pg_default_acl which stores initial privileges to be assigned to newly created objects.