How Do Water Heaters Work

I am just beginning to wonder the specifics of how normal water-heaters function in a house hold. I do not know much about them, I think ours holds 140 gallons, and is gas. How does it heat all of the water exactly and is not hot to touch the actual tank of stored water? How does it fill up with water (what records it is empty) and how fast does it fill up after being used and heat up? Also, how does it stay heated to remain available for anytime use? A few showers were taken, and I was last, where the hot water, of course, had ran out and it became cold, yet when I used to water from the sink it was able to get really hot (is it because the water is lower pressure?). And the last two things I am wondering is why it will heat up when you turn down the PRESSURE of the water but not change it to a hotter setting, along with how the temperature works with the handle for the shower. The more informative, the better, thank you!

The burning of gas is controlled by a thermostat to maintain an approximately constant water temperature. Cold water is routed through the water heater to be heated and then flows to the hot water faucets. The tank is always full. If the water is cooled by having cold water flow into it or by loss of heat through the walls of the heater, the gas comes on as needed to restore the set temperature. As water is heated, it naturally rises to the top of the tank where it is drawn off. That is why you found hot water at the sink soon after ending your shower.

There is insulation between the inner tank and the outer shell so that the shell is not as hot as the water.

The water temperature of the shower is a combination of hot water from the tank and cold water. The relative flow rates of the hot and cold that are mixed in the shower determines the temperature of the shower.

Changing pressure does not affect the temperature of the water in the tank. A pressure change of the cold water may affect the temperature of water in the shower unless a special type of shower valve is used that adjusts the hot and cold flows to maintain a set shower temperature.