Because your stupid! Call a plumber!
When you flush your toilet, cold water is used, this diverts cold water from the sinks and showers, resulting in diminished cold water flow momentarily at the sinks and showers, so, less cold mixed with hot, = hotter sinks and showers.
Sewage
Water is used in sinks, showers, baths, hoses, sprinklers, fountains, toilets, etc.
gray water
A stone sinks in water because it is more dense than water.
Tempered water is used for providing a safe and consistent water temperature in various applications such as showers, sinks, and dishwashers. It helps prevent scalding injuries by blending hot and cold water to reach a desired temperature range.
Cold water sinks, hot water rises.
Plumbing is the way to get water into our homes. You would have no toilets, baths, showers or kitchen sinks without plumbing - we would be little better than cave dwellers.
A single tankless water heater will not cope with all that.
Graywater is the waste water from such things as baths, showers, hand and kitchen sinks and sometimes washing machines. This water can be recycled to water gardens, for example. One can learn more about graywater online at sites such as Wikipedia, Graywater, and About.
Steam showers are a great addition to any bathroom. They work by water passing through a steam generator, located near the shower or inside the wall, and out through an outlet in the shower itself.
Outside at the main shut off valve. Showers/Bathtubs do not normally have a shutoff valve like sinks and toilets. The main water valve for the house must be turned off.