How does self-raising flour actually work?

A year ago, I got really into exercise and wanted to eat more protein, so I tried making a protein microwave mug cake. I could have misread the recipe, but I added both baking powder and self-raising flour, not realising that they both kind of did the same thing. So, as you can imagine, it was a disaster! The mug cake spilt all over the microwave and, whilst it was still yummy, it was partially raw. Speaking to anyone who had any experience doing anything had me realising that it was obvious they were conflicting ingredients. I don't think a recipe would have said that, so either I misread something, or I was an idiot (and likely still am). That past incident is sort of what inspired me to do this post in my 'The Baking of Biochemistry' series - the first one was just to look over tasting as a thing in general.

Every self-raising flour is made from plain flour with baking powder mixed in, and sometimes they have salt added for flavour reasons - I'm assuming because sometimes too much baking powder can make things bitter. Baking powder is a leavening agent (it's used in baking so things rise, soften, and become porous) and contains three key types of ingredients: an alkali, a weak acid, and a filler. The alkali used is normally a bicarbonate of soda (also known as baking soda); the weak acid tends to be dry acid (sodium bisulfate); and the filler can be something like cornflour or rice flour, and is added to prevent a premature reaction between the acid and alkali. The real reaction occurs when the baking powder is added to a liquid - so adding your dry ingredients to your wet ingredients - as this is when they can finally undergo an acid-base reaction (alkalis are also called bases). For anyone who has done GCSE chemistry, you may know (or might remember) that when an alkali and an acid react, one of the things they produce is carbon dioxide; this gas is what causes the mixture to expand and become porous. So it's recommended to put your mixture into the oven as soon as you've mixed your dry and wet ingredients. This will obviously also happen when self-raising flour is added to things, but if you've ever walked through a bakery aisle (not the one with the baked goods, the one full of ingredients), you'll know that baking powder (and baking soda) can be bought in their own separate containers. This kind of feels like an ingredient inception, and now I'm wondering if self-raising flour actually contains baking powder, which also contains baking soda. Kind of confusing. Especially since I use plain flour, baking soda AND baking powder when making chocolate chip cookies. Weird!

There were a lot fewer websites on this sort of thing than I expected, but I suppose it's something that's really random and irrelevant. But I think I managed to understand and somewhat relay to you how self-raising flour actually works, and it was much simpler than expected! Unless there was a much more in-depth study that was somewhere on the internet that I couldn't find, and these websites just lied to me.