Is this typical for how hot water heating loops are plumbed?
Yes, that is how modern hydronic heating systems are commonly set up. The main pipes are called the &quot;primary loop&quot; and each set of smaller piping to the radiators is called a &quot;secondary loop&quot;. (The technique has been around since at least the 1950's, but it used to be limited mostly to commercial buildings. Its use in residential systems has become a lot more common in the last 20 years.)
Boiler manufacturers want it done this way because the boiler is not designed to get back return water at too low a temperature. Yes, it sounds counterintuitive, I hear you thinking - isn't the whole point of the boiler to heat water? The problem is that if the temperature difference between the inside of the boiler and the returning water is too great, it will cause condensation inside the boiler which will ruin it.
Installers like to do it this way because it means you don't need to install lots of extra piping that would be required for a traditional &quot;header-type&quot; system with separate supply and return pipes.
It also means that you can turn on/off individual zones without affecting the overall system pressure too much.
If you want to know everything there is to know about Hydronic heating loops, Dan Holohan's website HeatingHelp.com is a great place to start. Here is their article on primary-secondary pumping. If you really want to go deep, Dan also wrote an entire book on the subject (as well as multiple other books about heating systems).