A boiler heats treated water under pressure. Steam evaporates from this heated tank and travels to an exit pipe/supply line at or near the top of the tank. The pressure of tank determines the temperature of the steam. Most boilers for brewing operate under 15psi, which means that they are and fall under low-pressure boiler regulations, and steam at 12psi is roughly 250 degrees F (google for steam temp charts). A temperature controller maintains the temperature of the boiler and the boiler is equipped with PRVs, VRVs (Vacuum), water level high and low and temperature limit detectors.
The steam follows the exit pipe to the consumer(s). The consumers either have a manual or a temperature/process controller-operated valve or both to control the inlet of steam to a jacket(s) on the vessel near the upper half of the jacket if it is a vertically positioned jacket. The jack should have a PRV and VRV. When the steam comes in contact with the wall of the jacked it dumps its heat on the jacket, condenses and falls to the bottom/low point of the jacket where a condensate outlet is located.
The condensate outlet should have a steam trap (bucket steam traps are common, reliable and cheap) - which allows condensate to pass but not steam. It is possible to operate without this, but its not particularly safe (steam exiting to work-place) and its extremely wasteful (energy loss). The trap outlet either drains to the environment, again very wasteful, or goes to a re-circulation tank which collects condensate and by a float-operated pump returns the condensate to the boiler tank.
All of these pipes and jackets should be insulated to avoid being wasteful - very wasteful - and exposing hot surfaces to flesh.
I've left out supply line condensate handling, particulate filters/traps & check valves, proper installation of all these parts, weep lines, supply & return line sizing and blow-down and periodic maintenance and testing procedures. All of this is important for safe and efficient operation of a steam boiler & system and entire books are dedicated to the science of steam - and this is just my limited knowledge.