UPDATE Number_Codes
SET
   CODE1 = ISNULL(CODE1, 999)
,  CODE2 = IIF(CODE1 IS NOT NULL AND CODE2 IS NULL, 999, CODE2)
,  CODE3 = IIF((CODE1 + CODE2) IS NOT NULL...

Here's another way:

DROP TABLE IF EXISTS #Products;

CREATE TABLE #Products
(
  Product VARCHAR(50) PRIMARY KEY CLUSTERED
, Type1 VARCHAR(50)
, Type2 VARCHAR(50)
, Type3 VARCHAR(50)
, Type4 VARCHAR(50)
);

INSERT #Products

This gives the same accuracy as your first 'exact' example:

SELECT CAST(1000000000000.123456 AS DECIMAL(29,12)) * CAST(10000.123456 AS DECIMAL(29,12))

This code helped me determine the CASTs to...