Around here, there are companies that specialize in replacing old water heaters. They are not plumbers and do not always do a professional job. Here is a picture from an older house, with a newer, but still old, water heater that has replaced the original.
Notice that the “installers” replaced the water pies close to the water heater with copper (common enough) and installed the required di-electric unions between the copper and the galvanized.
BUT… They did not replace all the galvanized pipe and just connected the copper back to the galvanized but without additional di-electric unions.
Another example of people just doing rote installations without even knowing or thinking about exactly what they are doing.
