The network includes, at any given time, all relevant isotopes from hydrogen through polonium (Z=84).
Even the limited grid of stellar masses studied suggests that overall good agreement can be achieved with the solar abundances of nuclei between model and (so far, only in that model) are a consequence of the merging of the oxygen, neon, and carbon shells about a day prior to core collapse.
While the abundance predictions have traditionally been used to fix the correct value for eta, there are different possibilities for measuring that number.
Most notably, the presence of particles like protons and neutrons in the early universe leaves a slight, but measurable imprint on the cosmic background radiation.
From about one second to a few minutes cosmic time, when the temperature has fallen below 10 billion Kelvin, the conditions are just right for protons and neutrons to combine and form certain species of atomic nuclei. While the early universe is totally unlike our everyday world, the basic nuclear physics at the appropriate energies is well within the range of laboratory experiments.