How to expand my LOW singing range?

Your lowest notes depend on the anatomy of your body, more specifically, the size and shape of your vocal cords. You can not do much about it.

For an analogy, imagine a guitar. You can loosen a guitar string and that will lower the pitch it makes. At a certain point, the string reaches its limit and can’t get any slacker. The pitch won’t go any lower because the string loses the tension and volume.

The same happens to your body. There’s a physical limit to how thick and slack your vocal cords can get.