I've been in plumbing for a year and a half, doing new construction in large apartment buildings here in NYC but don't have much experience yet with heating systems other than large fan coil units and only set up the radiators, so the logistics in the mechanical room still confuse me a bit.

I watched this video and I was wondering how the water 'knows' which pipe to go through? He put zone valves on the return lines, but wouldn't he have the same problem he describes in the beginning, that the water won't go up the new 1/2'' line due to water choosing the easiest path? All he did was put valves on the returns. There's still only 1 line coming out of the circulator pump, supplying 3 different lines.

I understand he installed thermostats but wouldn't the smaller line still perform poorly in comparison to the other radiators, due to water "choosing the easiest path"?

Shouldn't the system be pressurized?