In helium production, a proton and a neutron initially collide to produce a deuterium (an isotope of hydrogen composed of 1 proton and 1 neutron, H²) and energy (a photon). Next, two deutrium nuclei collide to produce an isotope of helium (He³) composed of 2 protons and 1 neutron, and 1 excess neutron. In the final step, He³ may capture a deuterium to make 1 helium (He⁴) and 1 proton or two He³ may merge to form a He⁴ and 2 leftover protons.

How this process ends is sensitive to how fast the above interactions can take place and to the temperature of the Universe

If the Universe is too hot, Step 1 can take place, but before Step 2 ensues, the deuterium breaks apart and no complex elements are produced.
If the temperature is right for the above to occur, but the density is very low, then although it is allowed, Step 1 will not occur and we will not produce complex elements
If the temperature is right and the density is higher, but still small then Step 1 will occur but Step 2 will be rare and we will wind with a lot of deutrium in the universe
If the temperature is right and the density is high, then we will wind up with a lot of He⁴ and essentially no deutrium in the universe