We understand this one surprisingly well. The seminal paper from 1957 is https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.29.547 https://journals.aps.o

We understand this one surprisingly well. The seminal paper from 1957 is https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.29.547 https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.29.... that figured out all of the reactions involved. Then used their predictions of the process to predict the distribution of observed isotopes of all of the elements. Those predictions were surprisingly accurate and have been refined more. See https://www.sciencealert.com/this-awesome-periodic-table-shows-the-origins-of-every-atom-in-your-body https://www.sciencealert.com/this-awesome-periodic-table-sho... for the outcome of that. Incidentally the photon takes so long to escape because it gets absorbed and reemitted over and over again in a drunken walk. A supernova is fast because most neutrinos just escape, but with enough being stopped to rip the star to shreds.