There is a slow build up. You need to contrast this with the situation in the earth's crust, where chemical properties play a large role in the abundances . Elements beyond the iron peak cannot be made by processes that can act as a source of stellar energy. Elements lighter than iron, can fused to form heavier elements and at the same time furnish energy. This is the principle of the hydrogen bomb, and the energy currently supplied by the sun.
Heavier elements can only supply energy by fission .
This is the principle of the original uranium bomb. The answer is that they are made by neutron addition processes. The sites of these processes are surely stellar interiors. Here is such a plot. Just how these peakes come to be at these particular places will be discussed in a course offered in the winter term on the history of the chemical elements.