Carl Sagan has said several times that we are "star-stuff".

One instance can be found in Good Reads' Carl Sagan > Quotes > Quotable Quote:

The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.

Question: Was most of my nitrogen really made in the interior of a star during its collapse? Was my calcium and iron made there as well, and not (for example) in an expanding shell after a supernova?