The boiler on a espresso machine has various functions. Traditionally the boiler is made from copper and is used for hot water, steam and heating up the water used for the brewing process.
In most cases there was only 1 boiler inside the espresso machine, but nowadays you do see more often espresso machines which have a central steam boiler and a brewing group boiler. There is also a change of materials, traditionally boilers where always made from copper and brass. But now you do see more stainless steel boilers used in espresso machines.
But you do also need to seal off these espresso machine boilers. In the past it was more common to have a open boiler on one, or 2 sides. These sides where sealed off with a boiler gasket and a boiler flange. Sometimes in combinations with boiler rings to even out the pressure over the flange.
You have components like a safety valve, this is the last safety feature on the espresso machine. this will boiler valve will open up when the pressure inside the boiler will go over a certain pressure. The vacuum valve on a boiler is used when the boiler is heating up to release the “false” air out of the boiler. This valve will close once the pressure inside the boiler is high enough.