How to select functions that belong in a given extension in PostgreSQL?
All credit for this answer goes to LaurenzAlbe who hinted the solution in the comment. The query that lists functions created by an extension is

SELECT e.extname, ne.nspname AS extschema, p.proname, np.nspname AS proschema
FROM pg_catalog.pg_extension AS e
    INNER JOIN pg_catalog.pg_depend AS d ON (d.refobjid = e.oid)
    INNER JOIN pg_catalog.pg_proc AS p ON (p.oid = d.objid)
    INNER JOIN pg_catalog.pg_namespace AS ne ON (ne.oid = e.extnamespace)
    INNER JOIN pg_catalog.pg_namespace AS np ON (np.oid = p.pronamespace)
WHERE d.deptype = 'e'
ORDER BY 1, 3