Sometimes the price you pay for learning a thing is having your colleague, in the middle of the explanation, offer you an undergrad astronomy textbook so you can get up to speed. BUT IT'S WORTH IT BECAUSE NOW YOU KNOW THE THING
Anyway, here: You know how massive stars can make oxygen, carbon, etc, in their cores? (Short version: our Sun does hydrogen->helium but heavy stars can go up to iron.) Pretty much none of what gets out, and later becomes planets & us, comes from that.
When the core turns into iron & the nuclear burning stops working, the star collapses, & the shockwave that tears through it quickly burns up everything the star just spent all that time creating through normal fusion, & THAT’S where the heavy elements that get out come from.
There are other processes that make heavy elements, like death throes of lower-mass stars, and neutron star collisions (!!), but the steady burning inside high-mass stars gets kind of exploded over and doesn’t account for much in the end.
There’s more info on all this on @jajohnson51’s page here: http://www.astronomy.ohio-state.edu/%7Ejaj/nucleo/index.html …
(If you’re wondering which part of this I learned today, it was the part about how almost none of what’s made in the core of a massive star survives. I’d thought some of it would.)
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