builtinCatalog

The catalog the driver starts out with, before a database has said anything: the codecs it ships, the types those codecs name, and the converters that map them.

The dictionary is derived from the codecs rather than read, because the query that reads the catalog is itself a result whose columns have to be described, and until it returns these are the only types there are to describe them with. Every entry is replaced the moment the real catalog arrives.

This is the whole of what "built in" means, in one list, kept away from CatalogHolder - which has no business knowing that a converter exists.