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1. If the supplies and returns share a common main (i.e: MonoFlow/Single-Pipe Diverter Tees), the tees themselves afford resistance in the main equal to the resistance through the radiator (heater I believe is what you consider a radiator? I will assume so.)
2. If the supply is dedicated and the return is also dedicated (Two-Pipe supply/return) then the pressure differential across the two mains is what induces flow from supply to return (high to low pressure. Of course the return is on the suction side of the circulator and the supply is on the discharge, so ultimately the differential across the circulator is what does the trick.
The closer the two connections are to the circulator, the greater the pressure difference. Pressure "decays" moving away from the circulator.
3. If there are a pair of closely spaced tees, such as a primary-secondary system, the secondary (branch) circuit has to have it's own circulator. As you note, without any inducement to take the side-road, the main flow will continue through the main tees without affecting the secondary circuit. That is the essential characteristic of P/S piping, two circuits that do not affect one-another hydraulically but work independently.
Does that help?