It is common misunderstanding in Qigong and Taoist circles that basic abdominal breathing cultivates energy in the lower Dantian. Actually before the fusion of yin and yang, one is only working with the “Lower Field” rather than the lower Dantian. When one fills the abdomen with yang qi, it is only around the lower Dantian, rather than inside it. Only after yin and yang is fused together does the energy enter the Real Lower Dantian.
It is very hard to access the Real Lower Dantian and takes years of foundational work to reach this point. However with the right teacher one may reach the point of cultivating the elixir as yin and yang is fused together. This is the start of the elixir and the beginning of “immortality” as the energy cultivated can never leave you if you have reached this stage.
From the point of yin and yang fusing, there are further alchemical meditations to transform the energy in the Real Lower Dantian into the Golden Pill.