Baking soda vs baking powder: what's the difference?

Last week, I made a blog post on how self-raising flour actually works, and through researching that, I found out that baking powder actually contains baking soda. However, not only did I already have this blog idea planned out, but I also wanted to know what the actual difference was. If baking powder is just baking soda with extra steps, why are they bought differently? Why are they both required as separate ingredients when making chocolate chip cookies? Which is my aim for this blog post. I want to know how they're so different that they warrant distinctive names, but also why I feel like it's not common knowledge that one contains the other.

As mentioned in my previous post (assuming you've read it), both baking soda and baking powder are leavening agents, which means they're used in baking to cause dough or batter to rise by releasing gas (some other examples of leavening agents are yeast, air, and steam, depending on the baking method). Though baking powder technically contains baking soda, they are both chemically different, so one shouldn't be directly substituted for the other. 

Baking soda tends to contain pure sodium bicarbonate (it's also referred to as bicarbonate of soda) and is basic (or alkaline), meaning it requires acid to activate it; this means that baking soda is ideal to use when a recipe contains something acidic such as citrus juice, cream of tartar, or buttermilk. Baking powder also contains this, but also has one or more types of solid acid and a stabiliser/filler like cornstarch (normally used to prevent an acid-base reaction occurring during storage). Therefore, it doesn't have to be activated using acid and can just be added. Recipes may contain both if they contain an acid; baking soda can be used to offset the acid to ensure it can be leavened properly.

I mentioned before that baking powder and baking soda can't be directly substituted, but you can substitute one for the other if you have the correct measurements (and other ingredients). When I was looking into this, I found a conversion guide that I've attached, which you can find using this link.