So is the oil is injected directly into the manifold through this orifice or
is there some more complicated, separate mechanism for heating up the oil?

It is injected directly. You have to experiemnt a bit to find the sweet
spot....not too hot and not too cold.... where the oil will completely vaporize
and not burn. Plus the right size orifice is needed and this varies with each
airplane installation.
VL
(Mopar electric fuel pumps were great smoke oil pumps......cheap and usually
reliable)