Based on memories of asking “the lithium question” and how it was answered:
It seems counterintuitive to think of the core of a red giant star as a quiet, peaceful place, but it is a place where steady gradualistic processes predominate. Protium fuses into deuterium, deuterium into helium-4, three helium-4 into carbon-12, and so on.
A reaction must be ‘arithmetically’ possible: e.g., nuclei with X protons and Y neutrons fuse with ones with Z protons and W neutrons to form an unstable nucleus of X+Z protons and Y+W neutrons, which then gives off an alpha particle and becomes a stable nucleus of element X+Z-2…)
There is also a threshold energy for its creation, a minimum temperature and pressure below which the reaction will not occur. And when you’ve reached that threshold, there may well be other reactions that “it’s hot and dense enough for” that will happen alongside it. Some of these are endothermic.
Lithium has two isotopes which are ‘stable’ – i.e., non-decaying at temperatures and pressures below those of stellar cores. Helium-3 is also stable; tritium, hydrogen-3, has an 11-year half-life, short on the geologic time scale but quite long compared to the average random non-stable nuclide, whose half-life may be measured in microseconds – such as beryllium-8.
What unites these nuclides, and makes them relatively uncommon in nature, is that the temperature and pressure needed to create them in an “isotope nursery” stellar core is higher than the temperature and pressure at which they tend to break down into smaller nuclides. They can be produced in trace quantities below that – all the tritium on earth, for example, comes from reactions in the upper atmosphere. And of course once produced, they exhibit their characteristic stability in terms of nuclear decay. But for the ‘industrial level’ production in stellar cores, fuggidaboudit! Not happening! Temperatures conducive to producing lithium nuclides are even more conducive to breaking down litium nuclides as soon as they’re produced.
The catastrophic conditions inside a supernova, and even more so within the Big Bang, are different. Particles and nuclides are slammed together willy-nilly, with no time for reaction or decay before they are again slammed into something else, and eventually expelled in the enormous explosions. The results include a small ‘production run’ of the so-called ‘forbidden nuclides’. the ones that in gradualistic stellar-core conditions will break down as soon as formed.
Hence the confident assertion about lithium.