These types of boilers are most common because of their ability to deliver large quantities of steam. The large tube like structure at the top of the boiler is called the steam drum. You could call it the heart of the boiler. That's where the steam collects before being discharged from the boiler. 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.