The hundreds of tube start and eventually end up at the steam drum. Water enters the boiler, preheated, at the top. The hot water naturally circulates through the tubes down to the lower area where it is hot. The water heats up and flows back to the steam drum where the steam collects. Not all the water gets turn to steam, so the process starts again.
Water keeps on circulating until it becomes steam.
Meanwhile, the control system is taking the temperature of the steam drum, along with numerous other readings, to determine if it should keep the burner burning, or shut it down. As well, sensors control the amount of water entering the boiler, this water is know as feedwater. Feedwater is not your regular drinking water. It is treated with chemicals to neutralize various minerals in the water, which untreated, would cling to the tubes clogging or worst, rusting them. This would make the boiler expensive to operate because it would not be very efficient.