I wrote: > For a composite type, pg_type.typrelid links to pg_class and pg_attribute > entries that work much like a table.
Actually, you could reverse that: for a table, pg_type.typrelid links to pg_class and pg_attribute entries that work much like a composite type.
For both relations and composite types, there are pg_class and pg_type entries that (by convention only) have the same names and namespaces. They cross-link to each other via pg_class.reltype and pg_type.typrelid. The associated pg_attribute entries have attrelid matching the pg_class OID. The catalog entries for the two cases are pretty nearly indistinguishable except for pg_class.relkind. Again, it's only by convention that we consider that the pg_type entry is primary for a composite type but pg_class is primary for a relation.
Of course, a relation has some underlying storage (for most relkinds), and it will likely have associated entries in other catalogs that a composite type won't. But the core catalog entries are about the same.