your question is a bit wide, but here's a specific:
this is for zoned hot water radiator heat:
[the water returning from the radiators is cooler and not as stressful
on the zone valve.]
that's where you put the zone valve, on the cooler hot line returning
from the radiator loop.
this zone valve tells the water to stop circulating to the radiator
when the room thermostat opens when satisfied.