My house has a gas based water heater(geyser) on the outside. The plumber has connected it such that without the use of electric pump, just through pressure alone, hot water reaches the 1st and 2nd floor. How does this work?
The same pressure (provided by your local utility)that pushes the water out of the first floor faucet. It all requires pressure. The water heater provides very little resistance to the flow of the water.
The water coming to your home is under pressure. Typically about 60 psi.
Imagine a bucket with a lid and in the lid you have two holes cut. In one hole, you place a garden hose that fits snugly. You turn the water on and the bucket begins to fill. When the water reaches the top of the bucket, it will start to come out of the second hole.
Your water heater works the same way. The tank has two holes. One for water in and one for water out. The water coming into the tank will push water out of the outlet and up through your house because of the pressure.
Assuming your municipal water pressure is 60 psi, that's enough to lift water about 140 feet, so getting the water a mere 15 feet or so above the water heater only reduces the water pressure by about 6.4 psi.