Ok so when a manifold is not a single unit like a 4branch 4-2-1 or 4-1 some manufactors use a cast iron construction manifold which can come in either two or three parts.

The first part directly bolts to the head and and is basically a 90' angle the directs all 4exits from the head in the next part which normally 'L' shaped which joins the top half of the manifold which is connected to the head the other end joins either the begin of the exhaust system of the cat.

As you can see from the attached image the above is a cast iron unit which just pipes joining pipes which is bad for gas flow, where as the stainless steel tubular (as they are otherwise called) has pipes joining at ruffly the same points which aids gas flow.



Manifold designs are alot more complicated than just getting all the pipes from the head to join with the exhaust system. Big power gains can be seen from a change of manifold.