What process in Type Ia supernovae is theorized to create more than half of all the iron in the universe?

Answer

Runaway thermonuclear reaction fusing material into $^{56}$Ni

Type Ia explosions fuse most of their material into $^{56}$Ni in a runaway thermonuclear reaction, and the subsequent decay of this nickel is the dominant iron source overall.

What process in Type Ia supernovae is theorized to create more than half of all the iron in the universe?
Elementastronomystarsupernovanucleosynthesis