There's more than one way to plumb radiators. I may not have the terminology proper, but if you have a 'main' line, and then it T's off to go to the radiators, that is a special type of T that scoops some of the flow into the individual radiator. Another way it is sometimes done is in a loop, where instead of a 'main' line, it literally loops through each unit. With that type of a system, you cannot turn one off, since the flow goes through all, it would block them all, since no water could circulate.
Over the years, if the boiler saw fresh water added, every time that happens, you introduce some air into the system dissolved in the water, and that means some oxygen, which also means some rusting of the iron components. Ideally, the thing would never leak, and all the rusting would be over in the first week or so. That is the reason why boiler piping and radiators can last a long time...it should be a nice sealed system, with no constant source of oxygen.
Older boilers tended to be turned on and left on during at least the heating season, and maybe all year if they also provided the hot water to the house either through an internal coil or an indirect WH. The older ones were not designed to be cold started frequently, thus they may bounce between a min/max setting. OFten, the only thing the thermostat did was turn the circulating pump(s) on or off. Newer boilers can have more sophisticated controls, and many them can safely do a cold start, and may not maintain any nominal temperature setting.
On the boiler itself, it will have some safety features: over temp, over pressure, minimum pressure; along with a high and low temp aquastat.
Not sure if this helps, but not sure what you are looking for.