These are copper press fit connectors. They have "o"rings installed at each end of the connector.
Once the connectors are in place, they are crimped with a special, and expensive, tool. They are supposed to be as good as sweating and Sharkbite connectors but they are not reusable.
Those fittings are crimped on with a very expensive machine and are good if done properly.
If not done well they leak straight away and I had to have the plumber come back to do a few on stainless pipe...
Water is apparently leaking from the bottom fitting of the galvanized union and causing corrosion of the galvanized steel pipe. Remove the union, clean the threads , apply pipe dope ( I prefer teflon tape) and reassemble tightly to prevent future leaks. The rusted pipe could be wire brushed clean and will not rust if kept dry.