Where in pg_catalog is the database or catalog information stored about relations?
You can't it's hard coded. The _catalog columns in the table schema literally cast from a call to current_database()

CAST(current_database() AS sql_identifier) AS table_catalog,


You can see them defined in the source code postgres/src/backend/catalog/information_schema.sql



The docs on information_schema_catalog_name make it explicit


  information_schema_catalog_name is a table that always contains one row and one column containing the name of the current database (current catalog, in SQL terminology).


The docs on information_schema.tables also say,


  Name of the database that contains the table (always the current database)