I own a small building with 3 apartments and 3 separate gas boiler hot water
radiator heat systems. Each boiler has a circulator pump that is attached
to a pipe that goes into the side of the boiler about half way down.
Another pipe comes out of the top of the boiler.

I am assuming that where the circulator pump is located is the return water
line that goes back into the boiler and the pipe coming out of the top of
the boiler is the supply/hot water line that sends hot water up to the
radiators to supply them with heat.

Is that correct?