What does it mean that hot and cold water pipes must be connected in series in decoration?

Tell a joke: I once helped a client decorate. Originally intended to use air energy (put it on the balcony, so there are hot and cold ports, and then connect it with two corner cooling valves and high-pressure pipes). Later, customers replaced air energy with electric water heaters and liquefied gas (both of which have hot and cold ports). Later, when I took a shower, I found that there was hot water in the whole room, and the bath water was getting colder and colder. When I called for maintenance, I found that the angle valves of the air energy hot and cold ports were all open, that is, hot water.