Soap is fatty acids reacted with a base to produce glycerin and long chain molecules that attach to both water an oil. The chains strip oils and the glycerin moisturizes. The problem is that they remove this glycerin in most soaps and resell it for industrial use.
The rest is usually detergents and foaming agents, which are synthetic compounds that are very good at stripping all oils off of a surface. This is why you need to use conditioner after shampoo.
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